FEATURE: whitelist theme repo mode (experimental)

In some restricted setups all JS payloads need tight control.

This setting bans admins from making changes to JS on the site and
requires all themes be whitelisted to be used.

There are edge cases we still need to work through in this mode
hence this is still not supported in production and experimental.

Use an example like this to enable:

`DISCOURSE_WHITELISTED_THEME_REPOS="https://repo.com/repo.git,https://repo.com/repo2.git"`

By default this feature is not enabled and no changes are made.

One exception is that default theme id was missing a security check
this was added for correctness.

app/controllers/application_controller.rb

@@ -414,7 +414,9 @@ class ApplicationController < ActionController::Base
     end
 
     if theme_ids.blank? && SiteSetting.default_theme_id != -1
-      theme_ids << SiteSetting.default_theme_id
+      if guardian.allow_themes?([SiteSetting.default_theme_id])
+        theme_ids << SiteSetting.default_theme_id
+      end
     end
 
     @theme_ids = request.env[:resolved_theme_ids] = theme_ids