Split models.py into several files

locations/models.py is approaching 2000 lines long. Move it to a models directory and split into several files.

Most models get a file, with some similar models grouped together in the same file. Use init.py used to maintain the same import structure.

The first commit is just models.py copied into several files with trimmed imports, to make it easier to rebase. The second commit is the changes required to make this work (notably adding app_label = 'locations') as well as pep8 updates. They will probably be rebased together when committed.