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
refresh_stats: Add some additional prints to report progress
refresh_stats: Marginal performance improvement on large projects
Stats: Add fields to cache last created unit and last submission

This will improve performance when retrieving the stats table.
Stats: Update last created unit

Current approach will increase load when adding whole new stores,
translation projects and projects.
Stats: Update last submission
refresh_stats: Update last unit and last submission fields
Stats: Update and retrieve cached unit and submission