Fix inconsistencies with last submission information

This commit does two things:
- Synchronizes existing submission information so that denormalized fields
  in the unit will have the expected submitter data.
- Prevents inconsistencies to occur again by setting the denormalized data
  within a `post_save` signal handler for `Unit` objects.

Fixes evernote/pootle#33.

Pick notes: Moved the code in the migration to upgrade.

Conflicts:
	pootle/apps/pootle_store/models.py
Use links to translation projects in the projects pages

Fixes evernote/pootle#40