publish fdroid client to github releases

In the course of mapping out #402 the idea of publishing F-Droid client to github releases came up. The rationale is that github is expensive to block (collateral freedom) and users can fetch F-Droid client there when f-droid.org is blocked for them. Additionally this is going to be a new feature of fdroid deploy and will allow app developers to publish their apps to their apps to own F-Droid repository and github releases with one call to fdroid deploy.

Of course Github being non-free, this is not exactly on brand for F-Droid. So I invoke lazy consensus. @fdroid