Improve admin interface filtering/navigating/querying

[matthijskooijman]

Now that Arta is out in the open, we'll need to improve the backend a bit so we can more easily keep an eye on the registrations. Here's some thoughts:

• Make foreign keys clickable (e.g. allow clicking user names in the registration changelist to jump right to that user's change page). Here's some suggestions on how to achieve this: https://stackoverflow.com/questions/6418592/django-admin-linking-to-related-objects The last answer looks like the most complete option (does require a dependency), though it is not quite clear to me if these link fields are always added at the end, or can be included in list_display as normal.
• Allow filtering Registrations on their options. This should probably do name-value based filtering (so chunking different options for different events with the same name). One question is how to handle fields and options with the same name, but a different title, but maybe just show the names?
• Show statistics for selected (filtered in the changelist) Registrations, i.e. show counts of options. This could maybe be an action that shows a statistics page (e.g. filter -> select all -> show stats), though a live side panel in the changelist would be more convenient.
• Make display of registration options a bit easier. The changelist for a registration already has a reasonably readable edit view for the options, but a more concise read-only view could also work.

This relates somewhat to #15 and #31.