Improve available frontends registry
Talking about this: https://git.pleroma.social/pleroma/pleroma/-/blob/develop/config/config.exs#L739
- First and foremost it should allow multiple refs, not just one, in case admin wants to install develop version
- refs should have their own meta information (i.e. version requirement, tags (stable/unstable/experimental), etc.)
- We should probably move this configuartion away from config.exs and instead have a separate repo (much like https://git.pleroma.social/pleroma/emoji-index ), where FE devs can contribute manifests for their FEs in a simpler (JSON/YAML) format for it to be aggregated.