"Add to collection" button at event

[tpronk]

Is your feature request related to a problem? Please describe
Adding events to collections now goes via the collection page. However, I find this a bit cumbersome as I cannot see any details on the event I'm adding there.

Describe the solution you'd like
I'd like an "Add to collection" button on an event page, so that I'm sure that I add the right event. Also it's handy when browsing events for those I might want to add to my collection.