Add Option tray now uses generic component

To test this, just click on the Add Option header link on the Config page. Should work as before.

Closes COUCHDB-2453
Hey @benkeen I really like this code reuse. But I also really dislike the `initTray` idea. 
I think you can do something like [this instead](https://github.com/apache/couchdb-fauxton/blob/master/assets/js/plugins/cloudant.pagingcollection.js#L54-L72). That way you can pass in the `toggleTrayBtnSelector` on standard construction and not have to have another method to initialize.