add migration for increasing title from 32 to 64 chars

- update initialization on import
- update validation rules

I created this migration to update the length of the "title" column on the p3_media table. This allows uploading images with longer file names and they will not be cut off after 32 chars.

In the migration I had to deactivate ForeignKey checks when I drop the p3_media table for renaming - since I have a number of constraints to this table. Not sure if you want to include that, but this might still be useful as a starting point.