Added token-based personal iCal feed

Gijs Hendriksen
2018-06-13T22:00:46+02:00

Closes #646

### Short description
Adds the option to download a personal iCal feed, which only shows the events you are registered for (and events for which registration is unnecessary).

### Steps to use this feature

1. Go to the `events` page.
2. Notice there is now another button for an iCal feed, containing a personal token in the URL.