Configure maximum time for feedback on server and validate feedback time

[ahus1]

Currently the frontend pulls information from the backend how long the feedbackup should be allowed. In ticket dukecon/dukecon_pwa#40 the frontend will adjust its UI to show the feedback button accordingly.

In this ticket the server should validate if the feedback for this conference is still in the allowed time frame. In order to do so it will need to have the configured time (this should be moved from the standard dukecon server) and it will also need the talk data (at least the end time).