Changeable frontends MVP

The frontend bundles MR at !2400 (closed) is getting too large and includes too many policy decisions that i'm not happy with. It also breaks existing frontend overrides and contains no tests.

This MR pulls some of the key ideas and parts of the bundles MR in, but in a minimal way so we can get it merged and think about how to get the rest in.

What this does:

• Make it possible to set replacements for the primary frontend in the configuration, which will get served out of a subfolder in the instance static path

What this does not:

• download stuff
• build stuff
• contain knowledge about the different frontends
• set admin or mastodon frontends

This MR will also not break existing overrides that people already have, and does not remove bundled frontends for now.

I'll merge this as the baseline of the feature so that we are not blocked any more. I'll open a separate issue to reintegrate the more advanced features of !2400 (closed) in a more controlled and easily reviewed manner.