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

lib/staff_message_format.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # This is used for formatting Suspension/Silencing messages.
 # It can be extended by plugins to provide custom message formats.
 class StaffMessageFormat
@@ -15,7 +17,7 @@ class StaffMessageFormat
 
   # Overwrite this to change formatting
   def format
-    result = ""
+    result = +""
     result << @reason if @reason.present?
     result << "\n\n#{@message}" if @message.present?
     result