Adds missing migrations

Some changes were made in the models in a branch different from the one that
introduced South in the project. After the merge, we didn't add migrations for
them. I tested this migration with a copy of the demo database in my localhost
and it seems to work fine, so we dodged a bullet here. We need to be more
careful about changes in the models.
Validates uniqueness of corpus name for one owner

This model will have to change if we intend to have group permissions for
corpora.
Merge branch 'bugfix/unique-slug' into develop