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/theme_javascript_compiler.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 class ThemeJavascriptCompiler
 
   module PrecompilerExtension
@@ -147,7 +149,7 @@ class ThemeJavascriptCompiler
 
   def initialize(theme_id)
     @theme_id = theme_id
-    @content = ""
+    @content = +""
   end
 
   def prepend_settings(settings_hash)