performance

Two fixes for performance. The first improves general performance while opening F-Droid and generally doing stuff (e.g. performing searches). It does so by removing a join onto fdroid_apk in the main query which fetches apps. The only join required onto that table is for getting the suggested app. The additional join ont fdroid_apk just to query information about the versions available was never used, because such information was never queried. This more than halves the query times on my device when both the main and archive repo is enabled.

The second fixes severe jumpiness while scrolling through search results. It does so by figuring out whether an app is media or not at the time that the repo is updated, rather than when scrolling the list. This means we can avoid hitting the disk and doing an expensive operation for each row in the results list. Note that this will likely still be jumpy when a large repo of only media is used.

Also includes a cursory fix for #1156 (closed) which was required in order to test whether this worked as expected.

Fixes #1143 (closed), #1156 (closed).