initdb: Create directories before creating essential users

This fixes the issue of Directory.objects.root being called before
other models are loaded
Accounts: Use settings.AUTH_USER_MODEL in PootleProfile as well
Accounts: Get rid of is_staff everywhere
Accounts: Add a custom User class
Accounts: Use full_name instead of first/last name
pootle_store/views: Simplify user/profile logic
Accounts: Get rid of PootleProfileManager
Accounts: Get rid of the custom Profile admin
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.