MastoAPI transition: composing new post and media upload.

• 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.

No known limitations as of right now, sorry.