Add separate version create view and create view URL
This is not currently used outside the new theme, but enables the
solution for #6238 -- latest `master` of the new theme already expects
this code.
For now, the create URL does throw and exception, because we are
rendering template code inside the version form. I'm not going to
address this as the create view is not linked and unused, and we
probably shouldn't be rendering templates inside form instantiation
anyways.
The edit version URL does change with this, to append the `/edit/`
postfix. This was done to match similar URL patterns, and to give a path
for the create view URL that does not match the edit URL pattern.
