FIX: Disallow editing of remote themes (#11189)

Allowing the editing of remote themes has been something Discourse has advised against for some time. This commit removes the ability to edit or upload files to remote themes from Admin > Customize to enforce the recommended practice.

app/controllers/admin/themes_controller.rb

@@ -299,6 +299,10 @@ class Admin::ThemesController < Admin::AdminController
     raise Discourse::InvalidAccess if !GlobalSetting.allowed_theme_ids.nil?
   end
 
+  def ban_for_remote_theme!
+    raise Discourse::InvalidAccess if @theme.remote_theme
+  end
+
   def add_relative_themes!(kind, ids)
     expected = ids.map(&:to_i)
 
@@ -357,6 +361,7 @@ class Admin::ThemesController < Admin::AdminController
     return unless fields = theme_params[:theme_fields]
 
     ban_in_allowlist_mode!
+    ban_for_remote_theme!
 
     fields.each do |field|
       @theme.set_field(