views.py: Extend MembershipCreate with org preselection
views.py: Extend MembershipCreate with org preselection

MembershipCreate's URL has been modified to allow for using organization
domain.  In this commit, the view is extended to save the organization
domain from URL and then retrieve the organization and supply it as
initial value to one of the form's fields.

vahtras/amy,swcarpentry/amy,vahtras/amy,vahtras/amy,swcarpentry/amy,pbanaszkiewicz/amy,pbanaszkiewicz/amy,swcarpentry/amy,pbanaszkiewicz/amy