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
* Doing it in a post migration was a bad idea
because the migration will fail if the site
is down while trying to download uploads
which points to the instance. This mainly
affects self-hosters using `discourse_docker`
where `./launcher rebuild` will take the
existing container down.
