Refactor routes/index as a class MainRouter

Also removed some methods/routes that were not being used and will have to
be refactored anyway if they are needed in the future. They are: get_join,
to join the session getting the username and password from the url, and
post_upload, to upload presentations from the html5 client (this was a
specially big method).
Refactor routes/socketio as a class WebsocketConnection

Most of its methods are used only internally, so now the class is more isolated
than before. But still with some methods that should be refactored.