Revise registration system (beta)

[ehmatthes]

I want to do as little as possible in alpha, so most of the registration system refinement is here.

• Support the following:
  • Allow people who find the site to request an invite.
  • Allow a site admin to issue an invitation in response to a request.
• Emails should come from this site.
  • Use SSSC address for emails?
• Move templates in accounts that are not allauth overrides to templates/accounts/, not templates/account/.
• Invitations will fail if a new invite has the same email username but different email provider as an existing user. Fix this so the new invite will have a unique username, and so the original user will not get a duplicate invitation.
• Shouldn't be able to issue an invitation to a user who has already set their password. Detect this, and inform admin that the person has already set their password, and they can use the reset password if needed. Show their username? This could be done in a non-success alert message.