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/discourse.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'cache'
 require 'open3'
 require_dependency 'route_format'
@@ -261,7 +263,7 @@ module Discourse
 
   def self.base_url_no_prefix
     default_port = SiteSetting.force_https? ? 443 : 80
-    url = "#{base_protocol}://#{current_hostname}"
+    url = +"#{base_protocol}://#{current_hostname}"
     url << ":#{SiteSetting.port}" if SiteSetting.port.to_i > 0 && SiteSetting.port.to_i != default_port
 
     if Rails.env.development? && SiteSetting.port.blank?
@@ -285,7 +287,7 @@ module Discourse
 
     return unless uri
 
-    path = uri.path || ""
+    path = +(uri.path || "")
     if !uri.host || (uri.host == Discourse.current_hostname && path.start_with?(Discourse.base_uri))
       path.slice!(Discourse.base_uri)
       return Rails.application.routes.recognize_path(path)
@@ -302,9 +304,9 @@ module Discourse
   end
 
   READONLY_MODE_KEY_TTL  ||= 60
-  READONLY_MODE_KEY      ||= 'readonly_mode'.freeze
-  PG_READONLY_MODE_KEY   ||= 'readonly_mode:postgres'.freeze
-  USER_READONLY_MODE_KEY ||= 'readonly_mode:user'.freeze
+  READONLY_MODE_KEY      ||= 'readonly_mode'
+  PG_READONLY_MODE_KEY   ||= 'readonly_mode:postgres'
+  USER_READONLY_MODE_KEY ||= 'readonly_mode:user'
 
   READONLY_KEYS ||= [
     READONLY_MODE_KEY,