Geocoding bug: Blocks must have *_from_num lower than *_to_num

[slinkp]

If a Block has left_from_num higher than left_tonum, you won't be able to geocode any addresses on the left side of that block. Ditto for right*.

• blockimport needs to check this on the way in and fix if necessary
• so does the admin UI
• we need a data migration that fixes this in existing databases; the boston demo has about 3% of its blocks broken

[slinkp]

(In [950e81beda5dd47862dd94eb159186600ebafb58]) Add Block.clean() method to avoid getting from_num and to_num out of order, plus a migration to fix existing databases; closes #164.

Also, slightly better admin form for Blocks, and enforce uppercase constraint on various Block fields.

[slinkp]

Ticket imported from Trac: http://developer.openblockproject.org/ticket/164 Reported by: slinkp