Merge suggestions (evernote/pootle#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

Pick notes: Made adjustments to the migration to match our models and to
    include the translation_project field that we also included in the
    new unique Suggestion model for showing contributors list. Also did
    other adjustments to alter code that Evernote no longer has. Other
    changes were necessary to fix issues with code that diverged.

Conflicts:
	pootle/apps/pootle_app/management/commands/refresh_stats.py
	pootle/apps/pootle_profile/models.py
	pootle/apps/pootle_project/models.py
	pootle/apps/pootle_statistics/models.py
	pootle/apps/pootle_store/models.py
	pootle/apps/pootle_store/views.py
Followup: rely on Suggestion's translation_project field

We kept it to simplify contributors list calculation.
Followup: Add missing logging

This is from a not picked Evernote commit.