Remove primary membership fields from the view

These fields have caused confusion as they don't get populated
automatically when entering data via the API and they don't work
properly when only adding a place and not a constituency.
Remove logic for processing primary memberships

This slightly confusing logic tried to find an existing organization
which the primary membership was referring to, and if it didn't exist it
created it. This is no longer needed because we're removing the concept
of primary membership.
Simplify membership removal javascript

We no longer need to worry about the primary membership being recreated,
so we can simplify the javascript which handles this.
Only name field is required for completed header

When the name field has been filled in we display the header which
allows the user to save and add another person.