Send invitation email when an existing Maptio user gets invited into a new organisation

[rgoj]

A separate issue for the below small part of #685:

In the case handled in https://github.com/Maptio/maptio/pull/674 (see description above), shouldn't we be sending some kind of an invitation to the user? Some notification that they've just been added to another team? Or perhaps at least let the admin know that no notification will be sent?

Creating this now as we've finally had a customer reporting this issue, so I'd like to have a place to track these reports for prioritisation: https://app.intercom.com/a/inbox/q3x5lnhp/inbox/shared/all/conversation/106323200015658#part_id=comment-106323200015658-21895731157