Improvements to Document type filtering in admin

* Tidies up the select field by grouping the sub-types in optgroups and
listing them at the end, after the main document types.
* gets rid of the auto-complete search for this select as it is short
enough not to warrent it.
* Simplifies the way Admin::EditionFilter filters by sub-type by using
the sub-type's id rather than the plural name.