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
"Rejecting" a user in the queue is equivalent to deleting them, which
would then making it impossible to review rejected users. Now we store
information about the user in the payload so if they are deleted things
still display in the Rejected view.
Secondly, if a user is destroyed outside of the review queue, it will
now automatically "Reject" that queue item.
