Bugfix: exclude must be a tuple

Fixes the bug:

    LanguageForm.Meta.exclude cannot be a string.
Bugfix: Return the right blank data in _get_checks()

Otherwise we might get a traceback in
pootle_store.views.py:get_qualitycheck_stats.
Accounts: Add a custom User class
Migrations: Update all migrations to point to accounts.user
Migrations: Also check for the presence of pootle_app_pootleprofile table
Migrations: Do not create a profile_id column in PermissionSet
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.
Accounts: Move pootle_profile.api to accounts.api
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)