1605 recruitment calendar views

This adds calendar views based on the fullcalendar implementation from !1348 (closed). However, these views are supposed to show free slots in recruitment rather than booked slots in execution.

There are still many open questions/tasks:

• The fullcalendar week view cannot display arbitrary times, you need to pick a resolution. I picked 15 minute slots from 9 to 17. We should check with the users. Maybe this needs to be configurable per institute. If we are out of luck this could also be a dealbreaker.
• The calendars are not updated automatically. You need to refresh manually. We could add some polling mechanism. Alternatively we could think about closing the popup automatically at some event.
• I was not very inspired about how to include the calendar links in the form UI. There is probably room for improvement.
• Some code is really hacky, but it does the job.
• I extended the nochrome option introduced in !888 (merged) so that it can be triggered on any view by a GET parameter ?nochrome=1. We should double check if this is a good idea. I have no specific guesses, but I smell security issues.
• I did not add corresponding feeds for the calendars. Is that necessary?
• I am not sure where the calendar code should live in the long term as we need calendars in different apps. For now I concentrated on the recruitment app. We probably have to make this more generic at some point.
• There are currently no permission checks on the resource calendar as users might only have permissions inside studies. (see #47 (closed))
• I did not include any information in the calendar items other than start and end. This is because in this specific case we are not interested in the appointments themselves but in the empty spaces between them.
• Ideally there was a single calendar showing appointments for both study and resources so it would be easy to find empty slots. However we could not yet think of a good UI for that, so for now we stick with two separate calendars. We assume that the resources are used sparingly and in the common case you only have to check the study calendar.
• It could make sense to highlight the appointments that belong to the current ParticipationRequest. This would allow to easily find slots like "same time next week". However, this would mean we could no longer leave the view open across different subjects.