Add warning to non-starred & older releases

Device OS and version: Android 5.1
F-Droid version (in the About screen): 0.100.1

What did you do? (clear steps if possible)

Went into the app, looking around the different versions available for F-Droid. I have the latest stable version installed, then accidentally tapped on a newer alpha version.

What did you expect to see?

A confirmation dialog with the warning that particular version is not 'preferred' or whatsoever (don't know what the 'star' after versions indicate?), and asking confirmation from the user that they want to download & install this version. A similar dialog could/should be displayed when users attempt installing a version older than the one currently installed.

What did you see instead?

That F-Droid immediately started downloading (& installing?) the alpha release.

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