migration: 17->18: Drop the thumbnail linking table as well
migration: 17->18: Drop the thumbnail linking table as well

Since we're in a weak sqlite context, the table won't be emptied
through the foreign key constraint, causing the table to be in an
inconsistent state.

chouquette/medialibrary,chouquette/medialibrary,chouquette/medialibrary,chouquette/medialibrary