RegistrationsController#create re-renders the form instead of redirecting, so user doesn't lose what they've entered.
Took out RegistrationsController#update, since all it was doing was calling super.
Took out stubbing of user save, because I actually want to check that save is doing the right thing.

Rename instantiate! to build, no more raising in build, no saving in build, no seeding of aspects in build.

This reverts commit 1481056a.

This reverts commit fb70dc8c, reversing
changes made to 53fef63a.

Conflicts:

	pkg/fedora/diaspora-setup
	pkg/ubuntu/diaspora-setup
	public/stylesheets/sass/application.sass
	spec/models/user/attack_vectors_spec.rb

RegistrationsController#create deals with validation errors. Username, email, password now required on sign-up.

Remove RegistrationsController#new (which just called super, so it's not necessary).