Improved InfiniteScrollRouteMixin

The fetch offset now depends on the current content length rather than
the previous fetch size
Added infinite scroll to stream search results

The twitch.tv API returns streams in a shuffled order sometimes.
All requests are correct though...
Maybe this feature needs to be disabled. I don't know yet...
Refactored itemHeight calculation of InfiniteScroll

Moved the static declaration of the item min-height to the stylesheets.
This makes it less ugly...