Added backend implemention for inviting new users + sending them an email. By default users who are invited are given the role of "Collaborator", as you'd want them to at _minimum_ collaborate with you.

Changed UserIdentity to prevent users PENDING_INVITATION from logging in.

Updates CSS and JS effects for user invitation.
Adds support for new role "collaborator" #89
Adds the ability to invite new users as collaborators #88 and gives them the ability to sign up through a custom portal
Adds new translations for aforementioned support