URSH: Validate custom shortcuts
URSH: Validate custom shortcuts

There shouldn't be any risk even if someone sends garbage containing
let's say `../`, but better to stay on the safe side than actually
sending an API request containing something obviously incorrect.

indico/indico-plugins,ThiefMaster/indico-plugins,indico/indico-plugins,indico/indico-plugins,ThiefMaster/indico-plugins,indico/indico-plugins,ThiefMaster/indico-plugins,ThiefMaster/indico-plugins