Migration: fixes postgress install

Store requires that pootle_app migration has already run and thus Directory is
available.  Other DBs seem to manage as they aren't as strict as Postgres. All
Postgres fans shout yay.
Profile: Get rid of PootleProfile.*_submission_count()
Accounts: Add a custom User class
Accounts: Use full_name instead of first/last name
Accounts: Implement User.display_name
Accounts: Move gravatar_url() to User model
Accounts: Get rid of PootleProfileManager
Accounts: Get rid of the custom Profile admin
Accounts: Move PootleProfile.get_unit_rows to User.unit_rows
Accounts: Move UserForm to accounts.forms
Accounts: Move User.post_save logic to accounts

This lets us avoid a call to get_user_model() before the app registry is ready.
pootle_store: Pass User instances instead of PootleProfile wherever possible
Store: Use user instead of profile in get_alt_src_langs()
Accounts: Get rid of some unnecessary calls to get_profile()
Store: Get rid of PootleProfile foreign keys (Part 1)
Store: Get rid of PootleProfile foreign keys (Part 2)
Store: Get rid of profile usage in templates
Stats: Use User instead of PootleProfile when calling stats
FIXME: Would this actually work?
Accounts: Get rid of request.profile