List events by tag

https://joindin.jira.com/browse/JOINDIN-504

Removed `SearchApi` class since it was duplicating functionality in `EventApi` for the sake of one URL parameter. `SearchApi->getEventCollection` is better handled by `EventApi->getCollection` and `SearchApi->getTalkCollection` was not even used anywhere (and could be handled by `TalkApi` if needed in the future). I can refactor back to using SearchApi if you would like to keep it around for some reason.

Tags are shown by the SearchController, so it's technically possible to combine tags with a keyword search, although there is no way to create that kind of requests in the UI currently (and I don't really know if that kind of thing is needed or usefull, but it's possible with the current implementation).

By the way, the placeholder text in the search form says "Search events or talks" but currently it's really not searching talks, only events.