Various bugs and fixes

Various bug fixes!

Various improvements and big-fixes.

fix: Force keeping old files to prevent dpkg failure on update (coder…

…#2399)

Updating a release if system files failed would result in failure from
the install script. This fixes it!

chore: convert citations to hyperlinks (coder#2392)

chore: add hero image to OSS docs homepage (coder#2241)

fix: Add tests for instance and app association (coder#2198)

This was regressed in coder#2187. There was bad testing around this
before, and this should prevent a similiar situation from happening
again!

fix: Improve Terraform agent<->resource association testing (coder#2187)