Syncing with core: Add obsolete field to Directory
Syncing with core: Remove translation_project field from Suggestion
Syncing with core: Remove uniqueness constraint in Suggestion

Allows to submit suggestions that have already been (maybe accidentally)
rejected.

Note that the `auto_now_add=True` of suggestion's `creation_time` field
has been adapted too. South was somehow setting this to `NULL` and this
avoids that weird behavior.

Pick note: picked from evernote/pootle@67e9a408 (only model changes,
the migration has been recreated).
Syncing with core: Drop TMUnit

Probably it is not necessary to remove most of the other code, and
maybe even some of it is left behind, but since we just want to
sync our models with core it is not so important.
Syncing with core: Drop our stats fields from TranslationProject
Syncing with core: Drop our stats fields from Store
Syncing with core: Add store field to Submission
Syncing with core: Add similarity and mt_similarity fields to Submission
Syncing with core: Add ScoreLog
Syncing with core: Add reviewed_by and reviewed_on fields to Unit

Note: This also removes auto_now_add=True on two other fields.

Pick note: picked from evernote/pootle@85025437 (only model changes,
the migration has been recreated).
Syncing with core: Add revision field to Unit