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/stylesheet/common.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'sassc'
 
 module Stylesheet

lib/stylesheet/compiler.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require_dependency 'stylesheet/common'
 require_dependency 'stylesheet/importer'
 require_dependency 'stylesheet/functions'

lib/stylesheet/functions.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module Stylesheet
   module ScssFunctions
     def asset_url(path)

lib/stylesheet/importer.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require_dependency 'stylesheet/common'
 require_dependency 'global_path'
 
@@ -34,18 +36,17 @@ module Stylesheet
     end
 
     register_import "theme_colors" do
-      contents = ""
+      contents = +""
       colors = (@theme_id && theme.color_scheme) ? theme.color_scheme.resolved_colors : ColorScheme.base_colors
       colors.each do |n, hex|
         contents << "$#{n}: ##{hex} !default;\n"
       end
-      contents
 
       Import.new("theme_colors.scss", source: contents)
     end
 
     register_import "theme_variables" do
-      contents = ""
+      contents = +""
 
       theme&.all_theme_variables&.each do |field|
         if field.type_id == ThemeField.types[:theme_upload_var]
@@ -66,7 +67,7 @@ module Stylesheet
     end
 
     register_import "category_backgrounds" do
-      contents = ""
+      contents = +""
       Category.where('uploaded_background_id IS NOT NULL').each do |c|
         contents << category_css(c) if c.uploaded_background&.url.present?
       end

lib/stylesheet/manager.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require_dependency 'distributed_cache'
 require_dependency 'stylesheet/compiler'
 

lib/stylesheet/watcher.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'listen'
 
 module Stylesheet