Revert templates to upstream and add our own to the custom directory

The upside of this is that keeping up to date with upstream is simpler, since it won't create merge conflicts. The downside is that if something changes in the upstream templates with respect to the information they have (variable name change, for instance), it will break our templates without warning.

One solution to the downside is to keep up to date with GOGS more regularly and follow the changes closely, instead of merging blindly.

We could also write tests that check if all pages render properly.

The issue names upstream templates and a custom directory but does not identify files, tests, or an entry point. Compare the upstream and custom template locations first, then determine how rendering can be checked; done means upstream templates are reverted, custom templates remain isolated, and pages render without breaking.

Written by the indexing model from the issue text.