Static frontend

This picks up from !882 (closed) and gets it to a usable state.

• Catches up with latest develop branch and fixes bitrot
• Adds permalinks to notice rendering
• Shows images, video, and audio, with NFSW filtering
• For notices that are part of a thread, shows all notices in the context

Of the tasks listed in !882 (closed), I felt like I am not qualified to address these remaining issues:

• Furling
• Consensus on how to override Pleroma FE
• Test strategy

I think the branch is useful as-is and could be merged to develop; these issues can be address in follow-up MRs. But if it's preferred (and if I can get some guidance on how to proceed) I could try to get them working in this MR.

This is the first Elixir code I have ever written, so some extra review may be needed; thanks!