MastoAPI transition: user profile settings

• Should use entitiy_normalizer wherever possible for consistency.
• Should not bring in regressions.
• Should not use extremely horrible hacks (up to reviewers to decide).
• Should not rename everything nor bring tons of refactoring to avoid merge conflicts and ease the review and testing.
• For every missing BE functionality there should be mention of it in pleroma#674 (closed), probably create a new task too.
• Code from mastoapi-exploration branch could be backported/separated or used as a reference.

As far as I know, MastoAPI simply doesn't have background field for a user, nor similar user profile customization, so setting that using standard API probably would be impossible. Same most likely applies for all the other settings that are permanently stored in BE (no_rich_text etc), so either those should be implemented in MastoAPI or left as is.