MastoAPI transition: user search

• Should use entitiy_normalizer wherever possible for consistency.
• Should not bring in regressions. (unless it's agreeable, see below)
• 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.

Soft-depends on !655 (merged), can be done in parallel but expect glitches without it. No known issues, sorry.