Allow a person to be invited multiple times.

Until they accept the invitation and they register.
Correctly display invited users to inviters.
Link in invitation email to source.

When user is invited, tell in the email where they were invited from.
Style.
Simplified invitations.

Now invitation is separate from action. User first has to invite and
then with next click do an action (like add as a maintainer). This now
allows inviting also on non-private publications.
Add _id to fields in observeChanges.
Use @ in events instead of template.data.
Document new field, invitedEmail.
Allow inviting users even when you have no permissions over the publication.
Better handling of sticky flash messages hiding.
Display an error for cancelled jobs as well.
Added TODO.
Describe the first link in invitation message.