DEV: enable frozen string literal on all files

This reduces chances of errors where consumers of strings mutate inputs
and reduces memory usage of the app.

Test suite passes now, but there may be some stuff left, so we will run
a few sites on a branch prior to merging

script/docker_test.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # This script is run in the discourse_test docker image
 # Available environment variables:
 # => NO_UPDATE        disables updating the source code within the discourse_test docker image