We want to display flash messages in admin when invites have been
cancelled. This message needs to display the user's email address, so
this commit adds endpoints to GET a single invited service user and org
user so that we can look up the email address of a cancelled user.
We want to start granting access to the org page. But it will be a bit
weird if the invites come from us personally, since the people we’re
inviting don’t know us.
It makes more sense, and sounds more official if the invites appear to
come from the ‘GOV.UK Notify team’ instead.
