When the feature is available, this setting allows admins to choose a
project as a source of custom file templates. This is in addition to
any instance-wide templates, whether custom or vendored into the GitLab
codebase.
This commit builds on the groundwork in
ee008e300b1ec0abcc90e6a30816ec0754cea0dd, which refactored the backend
so the same code could be used for new dropdowns. In this commit its
used for templates for the `.gitlab-ci.yml` files.