:sparkles: Implement Page bundling and image handling

This commit is not the smallest in Hugo's history.

Some hightlights include:

* Page bundles (for complete articles, keeping images and content together etc.).
* Bundled images can be processed in as many versions/sizes as you need with the three methods `Resize`, `Fill` and `Fit`.
* Processed images are cached inside `resources/_gen/images` (default) in your project.
* Symbolic links (both files and dirs) are now allowed anywhere inside /content
* A new table based build summary
* The "Total in nn ms" now reports the total including the handling of the files inside /static. So if it now reports more than you're used to, it is just **more real** and probably faster than before (see below).

A site building  benchmark run compared to `v0.31.1` shows that this should be slightly faster and use less memory:

```bash
▶ ./benchSite.sh "TOML,num_langs=.*,num_root_sections=5,num_pages=(500|1000),tags_per_page=5,shortcodes,render"

benchmark                                                                                                         old ns/op     new ns/op     delta
BenchmarkSiteBuilding/TOML,num_langs=1,num_root_sections=5,num_pages=500,tags_per_page=5,shortcodes,render-4      101785785     78067944      -23.30%
BenchmarkSiteBuilding/TOML,num_langs=1,num_root_sections=5,num_pages=1000,tags_per_page=5,shortcodes,render-4     185481057     149159919     -19.58%
BenchmarkSiteBuilding/TOML,num_langs=3,num_root_sections=5,num_pages=500,tags_per_page=5,shortcodes,render-4      103149918     85679409      -16.94%
BenchmarkSiteBuilding/TOML,num_langs=3,num_root_sections=5,num_pages=1000,tags_per_page=5,shortcodes,render-4     203515478     169208775     -16.86%

benchmark                                                                                                         old allocs     new allocs     delta
BenchmarkSiteBuilding/TOML,num_langs=1,num_root_sections=5,num_pages=500,tags_per_page=5,shortcodes,render-4      532464         391539         -26.47%
BenchmarkSiteBuilding/TOML,num_langs=1,num_root_sections=5,num_pages=1000,tags_per_page=5,shortcodes,render-4     1056549        772702         -26.87%
BenchmarkSiteBuilding/TOML,num_langs=3,num_root_sections=5,num_pages=500,tags_per_page=5,shortcodes,render-4      555974         406630         -26.86%
BenchmarkSiteBuilding/TOML,num_langs=3,num_root_sections=5,num_pages=1000,tags_per_page=5,shortcodes,render-4     1086545        789922         -27.30%

benchmark                                                                                                         old bytes     new bytes     delta
BenchmarkSiteBuilding/TOML,num_langs=1,num_root_sections=5,num_pages=500,tags_per_page=5,shortcodes,render-4      53243246      43598155      -18.12%
BenchmarkSiteBuilding/TOML,num_langs=1,num_root_sections=5,num_pages=1000,tags_per_page=5,shortcodes,render-4     105811617     86087116      -18.64%
BenchmarkSiteBuilding/TOML,num_langs=3,num_root_sections=5,num_pages=500,tags_per_page=5,shortcodes,render-4      54558852      44545097      -18.35%
BenchmarkSiteBuilding/TOML,num_langs=3,num_root_sections=5,num_pages=1000,tags_per_page=5,shortcodes,render-4     106903858     86978413      -18.64%
```

Fixes #3651
Closes #3158
Fixes #1014
Closes #2021
Fixes #1240
Updates #3757

1 files changed, 84 insertions, 0 deletions

diff --git a/hugolib/prune_resources.go b/hugolib/prune_resources.go
new file mode 100644
index 000000000..8eddafb53
--- /dev/null
+++ b/hugolib/prune_resources.go
@@ -0,0 +1,84 @@
+// Copyright 2017-present The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package hugolib
+
+import (
+	"fmt"
+	"io"
+	"os"
+	"strings"
+
+	"github.com/spf13/afero"
+)
+
+// GC requires a build first.
+func (h *HugoSites) GC() (int, error) {
+	s := h.Sites[0]
+	imageCacheDir := s.resourceSpec.AbsGenImagePath
+	if len(imageCacheDir) < 10 {
+		panic("invalid image cache")
+	}
+
+	isInUse := func(filename string) bool {
+		key := strings.TrimPrefix(filename, imageCacheDir)
+		for _, site := range h.Sites {
+			if site.resourceSpec.IsInCache(key) {
+				return true
+			}
+		}
+
+		return false
+	}
+
+	counter := 0
+
+	err := afero.Walk(s.Fs.Source, imageCacheDir, func(path string, info os.FileInfo, err error) error {
+		if info == nil {
+			return nil
+		}
+
+		if !strings.HasPrefix(path, imageCacheDir) {
+			return fmt.Errorf("Invalid state, walk outside of resource dir: %q", path)
+		}
+
+		if info.IsDir() {
+			f, err := s.Fs.Source.Open(path)
+			if err != nil {
+				return nil
+			}
+			defer f.Close()
+			_, err = f.Readdirnames(1)
+			if err == io.EOF {
+				// Empty dir.
+				s.Fs.Source.Remove(path)
+			}
+
+			return nil
+		}
+
+		inUse := isInUse(path)
+		if !inUse {
+			err := s.Fs.Source.Remove(path)
+			if err != nil && !os.IsNotExist(err) {
+				s.Log.ERROR.Printf("Failed to remove %q: %s", path, err)
+			} else {
+				counter++
+			}
+		}
+		return nil
+	})
+
+	return counter, err
+
+}