It used to allow adding email addresses to a group even if invites were
disabled for the site. This does not allow user to input email address
if they cannot invite.
The second thing this commit improves is the message that is displayed
to the user when they hit the invite rate limit.
This reduces chances of errors where consumers of strings mutate inputs
and reduces memory usage of the app.
Test suite passes now, but there may be some stuff left, so we will run
a few sites on a branch prior to merging
Bianca Nenciu
Sam Saffron
Guo Xiang Tan
Neil Lalonde
Guo Xiang Tan
Robin Ward
Régis Hanol
Gosha Arinich