Fixes #6510 and #6511
Catch the LookupError from get_supported_content_language_variant so that if the active language is not one that is recognised as a content language, the URL prefix will be left unchanged from the page's reported locale.
Additionally, skip the `translation.override` when WAGTAIL_I18N_ENABLED is false; this accounts for the fact that existing sites may have used alternative / custom i18n implementations involving i18n_patterns, and on these sites we can't rely on the page's locale field to be meaningful. That way, we restore the pre-2.11 behaviour: if the existing i18n code was using some kind of wrapper around the get_url_parts call, then that will work again; and if not, the URL will reflect the current active locale, which is no worse than before.
Fixes #6518
If the models.py containing a custom user model directly or indirectly imports wagtail.admin.auth, this causes a circular dependency because wagtail.admin.auth attempts to import django.contrib.auth.views, which in turn depends on the custom user model. Fix this by moving the django.contrib.auth.views import inside reject_request.
