Bundle the react dashboard code into the agent repo directly. (#11139)

* Remove code for bundling the dashoard on install.

* Bundle the dashboard code directly into the agent repo.

This diffstat looks huge, but it’s actually relatively simple. The only
_actual_ changes are in the Makefiles, `configure.ac`, and the addition of
`generate_dashboard_makefile.py`. Everything else consists of removing
files that are included in the dashboard tarball, and extracting the
contents of the tarball into `web/gui/dashboard`.

* CI cleanup.

* Automate bundling of the dashboard code.

This replaces the makefile generator script with one that handles
bundling of the dashboard code in it’s entirety, and updates the GHA
workflow used for generating dashboard PRs to use that instead of the
existing shell commands.

It also removes the packaging/dashboard.* files, as they are no longer
needed.