app detials: show either summary or changelog, depdening on whether the app is installed or not
This change improves the logic of displaying latest changelog entries (aka. the blue what's new block when looking at app details).
behavior:
- app not installed: changelog always hidden, app short-summary always visible
- app installed: changelog always visible, app sort-summary always hidden
(This is a follow-up to offline conversion with @eighthave and I'm basically implementing his suggestions.)
Edited by Michael Pöhn