Enable Admin-Javascript for plugins

Currently plugins can only enhance the end user frontend javascript through the "register_asset" function. As the admin-javascripts are loaded after the app-assets, Admin-Javascripts aren't defined yet and therefore can't be extended through plugins.

This Pull-Requests add the `:admin` option to register_assets to allow the specification of "admin-only" javascripts coming from the plugin which are loaded only after the normal discourse admin javascripts are loaded.

Includes the common tests on the plugin registry.