Add store field to Submission model

Cherry-Picked-From: 79c7e06f50c3393b6750901f13032789ee9d36c7

Pick notes: Had to update last_submission not to clash with the new fields.
Django 1.6: Replace get_query_set() by get_queryset()
Don't delete Store and Directory objects anymore

They'll become obsolete instead, thus allowing to keep units around, which
are used for keeping track of translator's activities or for generating
invoices.

Fixes evernote/pootle#269.

Cherry-Picked-From: b47cd81ac6ae06142de355f5bd0a529a88cd6a92

Conflicts: *.py

Fixes #3295