Patch all migrations to remove generic fields

The generic fields break with Django 1.6 and Mezzanine throws the following error if you try to migrate the mezzanine-page-auth app:

    Your project contains migrations that include one of the fields
    from mezzanine.generic in its Migration.model dict: possibly
    KeywordsField, CommentsField or RatingField. These migratons no
    longer work with the latest versions of Django and South, so you'll
    need to fix them by hand. This is as simple as commenting out or
    deleting the field from the Migration.model dict.
    See http://bit.ly/1hecVsD for an example.