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
* Our codebase currently supports custom interpolations keys that are
not present in the original translation. The proper fix should
be to make `TranslateOverride` aware of such keys.
