Replace ROLES_BASEROLES with db attribute and make base role membership implicit

migrations/versions/aff5f350dcdf_added_role_is_default.py

tests/test_invite.py

tests/test_role.py

tests/test_user.py

uffd/default_config.cfg

uffd/invite/models.py

uffd/invite/templates/invite/new.html

uffd/role/models.py

uffd/role/templates/role/show.html

uffd/role/views.py

uffd/selfservice/templates/selfservice/self.html

uffd/signup/models.py

uffd/user/templates/user/list.html

uffd/user/templates/user/show.html

uffd/user/views_user.py