FEATURE: Add auto update field to themes (#11102)

Themes marked for auto update will be automatically updated when
Discourse is updated. This is triggered by discourse_docker or
docker_manager running Rake task 'themes:update'.

lib/tasks/themes.rake

@@ -51,6 +51,22 @@ task "themes:install" => :environment do |task, args|
   end
 end
 
+desc "Update themes & theme components"
+task "themes:update" => :environment do |task, args|
+  Theme.where(auto_update: true).find_each do |theme|
+    begin
+      if theme.remote_theme.present?
+        puts "Updating #{theme.name}..."
+        theme.remote_theme.update_from_remote
+      end
+    rescue => e
+      STDERR.puts "Failed to update #{theme.name}"
+      STDERR.puts e
+      STDERR.puts e.backtrace
+    end
+  end
+end
+
 desc "List all the installed themes on the site"
 task "themes:audit" => :environment do
   components = Set.new