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

Dangerfile

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 if github.pr_json && (github.pr_json["additions"] || 0) > 250 || (github.pr_json["deletions"] || 0) > 250
   warn("This pull request is big! We prefer smaller PRs whenever possible, as they are easier to review. Can this be split into a few smaller PRs?")
 end