Use named ForeignKeys in migrations

[Elrond]

db/migrations.py has this:

from mediagoblin.db.models import (MediaEntry, Collection, MediaComment, User,
        Privilege)

This is a big NO! Those are the current variansts of those tables. Not the versions needed in any old migration. Those current tables could have changed in a completely unexpected way and break old migrations.

If there are migration docs, please add a note about this.

The current main use of the above imports is code like this:

creator = Column(Integer, ForeignKey(User.id), nullable=False)

This can be easily fixed:

creator = Column(Integer, ForeignKey('core__users.id'), nullable=False)

After fixing this, please make sure the migrations still work.