fix bug for groups/invitations#create (when not using js)

support invitation cancellation and invitation not found

cancellation, refollow invitaitions working

feature complete invitations system
Improve invitations system

- support invitation cancellation and invitation not found
- fix bug for groups/invitations#create (when not using js)

Conflicts:
	app/controllers/groups/invitations_controller.rb
Merge branch 'bug/cancel-rather-than-delete-invitations' of github.com:loomio/loomio into bug/cancel-rather-than-delete-invitations

Conflicts:
	app/controllers/groups/invitations_controller.rb
fix da specs
Merge pull request #767 from loomio/bug/cancel-rather-than-delete-invitations

fix problem where cancelling invitations deletes them