Add another suggestion field to submissions

Part of evernote/pootle#163

Pick note: Regenerated the whole migration.

Conflicts:
	pootle/apps/pootle_statistics/models.py
Add app suggestion fields to store suggestion

Part of evernote/pootle#163

Pick notes: Did some adjustments to model, including keeping the
    translation_project field, and completely regenerated the migration.
    Recovered SuggestionManager in order to provide the new `pending`
    method.

Conflicts:
	pootle/apps/pootle_store/models.py
Add missing logging

This is from a not picked Evernote commit. It is necessary to get working
one the migration in the next commit.
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.
    We didn't pick the changes for saving the store when creating
    Submission objects.

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.
Use Suggestion.reviewer as submitter in accepting and rejecting submissions
Cleanups
Followup: Use the right Suggestion model
Remove pootle_app.Suggestion model

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

Pick notes: Adjusted the migrations to match our numbering and our models.
    Did other minor changes due to code divergence.

Conflicts:
	pootle/apps/pootle_app/models/__init__.py
	pootle/apps/pootle_app/models/suggestion.py
	pootle/apps/pootle_statistics/models.py
i18n: Update an outdated docstring