Hooks: Get rid of hooks

These will be reintroduced as django signals should we ever need them again
Based on evernote/pootle@a5526c254e6e3c31ab23d20d2b81f9aafa94d8ee
Update lxml requirement to match TTK
Cleanup: removed dead code
Node might be of a text type and thus not have the `getAttribute` method

This fixes the JS error that occurred on permissions page
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