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
Remove pootle_app.Suggestion model

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