remove debug symbol
sitting: add a basic drop-down menu

1. fetch 'http://api-beta.ly.g0v.tw/v0/collections/sittings?q={"ad":8,"committee":null}&f={"motions":0}'
   to get a list of sittings for drop-down menu.
2. if url is /sitting, forward to the last sitting
3. if url is /{sitting id}, update model to match url
4. if user choose a sitting from drop-down menu, transit to
   corresponding sitting.