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
Locking a post prevents it from being edited. This is useful if the user
has posted something which has been edited out, and the staff members don't
want them to be able to edit it back in again.