Add support for mirrors to ease load on main f-droid repo

[mahdi1234]

The official client has support for using mirrors for index sync and app downloads. It would be nice to add it as well to offload the main server.

List of active mirrors can be found here - https://gitlab.com/fdroid/repo-monitor

[Surendrajat]

There is a much faster f-droid repo CDN mirror sponsored by Cloudflare at https://cloudflare.f-droid.org/repo
more info

[ElonSatoshi]

I've switched to F-Droid Basic because of the lack of mirror alternation in Neo Store and Droid-ify. I hope I can switch back some day.

[lepras]

bump, need this.