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

[achilleas-k]

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.