Fix PageUserAdmin for custom user model.

Don't crash at startup if the custom user model has no first_name and
last_name fields.

The PageUserAdmin still won't actually work, so you'll have to subclass
it and change the fieldsets to include the fields that actually work, but
at least you can import it now.
Modify the custom user model to have fewer fields.

Allows us to test whether PageUserAdmin works or not.
Fix tests throwing another exception during teardown.

This conceals the original error that you actually want to fix.
Test against custom user model to trigger failures.
Merge commit '2eb9e84' into fix_custom_user_model_admin

Conflicts:
	.travis.yml
Merge branch 'develop' into aptivate-fix_custom_user_model_admin
fix imports
fix custom username
further refactoring
fix some toolbar logout username handling
made the tests work with different user reqs
docs
Merge pull request #3483 from digi604/fix_custom_user_model_admin

Fix custom user model admin