Enable user to decide which mirror should be used

Currently, while the client supports mirrors and we can add them, they are only used when the main site fails. There is no way a user can select which mirror to chose (or even use a mirror at all) – except by blocking the main sites URL/IP on the device, in which case the website would be blocked as well.

I hereby propose to let the user select which mirror should be used – maybe even analogous to how it's handled e.g. by Linux Mint (performing a speed test and allow the user to pick the "speediest candidate"), though the latter would just be a "bonus point".

This wouldn't only make updates faster for users who picked a faster mirror – it would also lighten the load to the main site, distributing it between available mirrors.