This change both speeds up specs (less strings to allocate) and helps catch
cases where methods in Discourse are mutating inputs.
Overall we will be migrating everything to use #frozen_string_literal: true
it will take a while, but this is the first and safest move in this direction
This functionality was never supported but before the new review queue
it didn't have any errors. Now the combination of settings is prevented
and existing sites with sso enabled will be migrated to remove invite
only.