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
* 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.