Support for Django up to 2.2.

`textile` and `pipeline` dependencies are now in extras.
Deprecated `utils.urls.i18n_patterns` and `middleware.URLLocaleMiddleware`,
  as Django 1.10 adds `prefix_default_language` parameter to `i18n_patterns`.
Removed `auth_backends.AttrCASBackend`; use `django-cas-ng` instead.
Added this changelog.