Keep track of the state when changing pickers

Keep track of the state when changing the pickers to be able to share a link to a specific course/part/event series.

```
https://www.timetable.cam.ac.uk/#tripos/african/mphil
```
We might also need to listen to the `hashchange` event on the window object. This will allow us to update the content when you navigate through the browser's history.

```
$(window).on( 'hashchange', function(ev) {
    ...
});
```