WIP: Fixing mutes not persisting with Pleroma-BE

Fixes pleroma#211 (closed)

Changed the way mutes are stored - instead of storing them in user they are stored as a map screen_name -> isMuted in config, the same way color/highlight options are stored in !285 (merged). Reasoning behind this is that right now muting is stored in user and user itself is stored in a huge array of users alongside with a lot of other metadata (background URI, nicknames, etc), persisting all that data seems like a bad idea, especially since local storage is rather limited in terms of space, and storing only part of that (like, state.users.usersObject.*.muted or something) seems like a lot of extra work for no good reason.

In future, if both this and !285 (merged) get merged we probably should keep a separate config entry for all user-related customizations, instead of several separate entries.

Current implementation should work with BEs that support server-side mutes, but i haven't tested that - i can see LS taking priority over server-provided list while losing sync with it. Maybe i'll try doing blind theoretical fixes.

Testing on compatible BE is appreciated!