Merge suggestions (#163)

- Move all pootle_app.Suggestion data to pootle_store.Suggestion
- support new Suggestion schema where old pootle_app.Suggestion(s) were used
- Save reviewer when accept a suggestion
- Get pending suggestion for a Unit only
- Get translation_project for suggestions from unit joined with store
- Cleanups
- Update last action when accept and reject suggestions
Remove pootle_app.Suggestion model

remove from_suggestion field from Submission model
(last part of #163)