User: Inherit PermissionsMixin for our custom User model

AbstractBaseUser, which we inherit from, does not provide "has_perm". This is
taken care of by PermissionsMixin which only AbstractUser has by default.

Incidentally this gives us back is_superuser so we can get rid of it from our
class. No migration needed.
Keeping in mind it *also* brings us groups, which is not so great since we
don't actually use those anywhere.

Fixes #3286
Migrations: Fix another dependency
Migrations: Fix frozen models and complete_apps

This allows to create migrations again using:

schemamigration accounts --auto
Migrations: Alter migration to depend on migration 0002 from accounts
Migrations: accounts.User now inherits from PermissionsMixin

Followup for 4d678d8.
Migrations: Drop table for voting app

Followup for c977e615.
Migrations cleanup: Align frozen models to ease comparison
Migrations: Fix field in frozen models across migrations
Migrations: Add dependency
Migrations: Add another dependency
Migrations: Add another new dependency
Migrations: Add missing frozen models on migration
Migrations: Fix frozen models
Migrations: Fix more frozen models

Adding dependencies on other migrations allows to simplify the frozen
models. Also all the references to PootleProfile were changed in
favor of accouns.User. Other fields have been added or removed as
necessary.
Migrations: Permissions need to be adjusted

The permissions for pootle_app.Suggestion do use the contenttype for
such model, and since we are removing that model and its contenttype,
these references need to be properly adjusted.