Add button to trigger a repository update

Reason: The regular update check doesn't seem to work reliably (battery "optimisation" issue?), and the drag-to-reload mechanism is very hard to trigger on a landscape-only device (2160x1080). Alternatively you could make drag-to-reload trigger earlier.

F-Droid client version: 1.3.1 Android version: 7.1.1

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information