Support https downloads

[z]

Currently xonotic.co is only serving off http. The manager still verifies and warns the user if the is a hash mismatch between the database and the downloaded package. but an attacker could still attempt to MITM this. This is not any less safe than the Xonotic client itself's method for downloading maps.

[z]

This is supported on https://xonotic.co, basically a new release needs to go out to make it the default, or just update your ~/.xmm.ini