MastoAPI transition: notifications

• 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.

Right now MastoAPI is missing is_seen property present in QvitterAPI, it is mentioned in pleroma#674 (closed). This isn't the end of the world and could be easily worked around but it would bring regression of #380 (closed) Another possible regression could be that notifications in MastoAPI do not have "x favorited status y" and probably repeats are handled differently too, so Pleorma-FE won't increment post counters from notifications anymore automatically, but this is a such minor case it could be worked around separately if it's true.