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
This removes all uses of both `send` and `public_send` from consumers of
SiteSetting and instead introduces a `get` helper for dynamic lookup
This leads to much cleaner and safer code long term as we are always explicit
to test that a site setting is really there before sending an arbitrary
string to the class
It also removes a couple of risky stubs from the auth provider test
