Add a search function

[martin-ueding]

I just wanted to find a particular activity by name. It would be cool if we had a search function here that would allow some basic searching by name.

Then it would be even cooler to search activities by location or tile or something. Click on a map to select a position and then get search results. That might not be too hard with Leaflet.

[martin-ueding]

I've added a primitive search function for substrings in the name.