CMS_PERMISSION: optionally use raw id user lookups

The "view restrictions" and "page permissions" inlines on the "page"
admin change forms can cause performance problems where there are many
thousands of users being put into simple select boxes. Added a new
setting (CMS_RAW_ID_USERS) that, if set, forces the inlines on that page
to use standard Django admin "raw ID" lookups rather than select boxes.
The restriction on which users may be selected should still be honoured
by the new widget because we set the 'limit_choices_to' attribute to be
the same as the queryset on the original widget.

Using raw_id_fields in combination with limit_choices_to blows up if you
have many thousands of users. For this reason, we only apply this limit
if the number of users is relatively small (< 500, though that figure is
somewhat arbitrary). If the number of users we need to limit to is
greater than that, we use the usual input field instead unless the user
is a CMS superuser, in which case we bypass the limit. Unfortunately,
this means that non-superusers won't see any benefit from this change.