Fix legacy upgrade with missing instance

If the user is trying to upgrade their account and for some reason the
instance has been deleted in the meantime then we simply redirect to the
main listing of instances.
Add hashes to JS and CSS files for cache invalidation

Only do it for JS on production as requirejs makes this hard on
development and it shouldn't really be needed there either.

When I say it makes it hard I mean that because it's requesting lots of
files and then using the path of those files as the module name putting
in place the code to resolve this is both painfully manual and hence
error prone and also problematic because of the way require handles
fetching. Given cache busting is only really an issue with production
deployment it's not worth the pain.

For #554
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.