checksum personas to detect dupes (bug 641644)

> - Calculate hashes of final, full sized images (both header and footer) for all themes
- On upload, we calculate the hashes like normal and put them in the db.  New themes go in the review queue as normal.
- We alter the review queue with a place for a message to the reviewer, and if the hash is the same as one already in the db (please add an index to that column!), then a short message to the reviewer is all that is needed.  Something like "This theme might be a duplicate of _$themename_".  If it is a dupe, they should reject/delete it like any other disallowed material.