frontend install mix tasks

This is a different (and simpler) approach to the downloading / installation of frontends, similar to what was happening in !2400 (closed)

The simplification mostly comes from only supporting one style of frontend with the new mechanism (that is, no special rules for mastodonfe or any kind of generalization across possible future frontends). I don't think that was worth the complexity that was added. Long term, MastodonFE should be modified so that it can work without special information.

The only 'special knowledge' that pleroma has about frontends is a configurable information about available frontends in the config file. These can be easily installed without any additional information. For custom frontends, you'll have to give more command line options.

Automatic building is out for now as well, I don't want a dependency on yarn just yet.