Requirements: Update Tastypie requirement

Necessary to avoid some warnings, to get a lot of bugfixes and to
have a recommended version.
Accounts: Use settings.AUTH_USER_MODEL in PootleProfile as well
Accounts: Add a custom User class
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.