Add 'copy OS to USB' button to USB category

This adds support for copying the latest version of the OS to a given
location. The primary use case is for copying the OS to a USB disk to be
used to update the OS on another computer.

This functions fairly similarly to copying an app to a USB stick.

As with the ‘Copy to USB’ button for apps, only show it if it might work:
if there's a collection ID configured on the OS remote which is a
prerequisite for the copy to succeed.
1 job for usb-os-copying in 17 minutes and 1 second (queued for 1 second)
Status Job ID Name Coverage
failed #307753


Name Stage Failure
build-gnome-software Build

Ok: 0
Expected Fail: 0
Fail: 1
Unexpected Pass: 0
Skipped: 0
Timeout: 0

ERROR: Job failed: exit code 1