GUI: allow tourney owner to cancel a draft tournament

Sometimes a draft gets messed up, for example if 1-2 people suffer issues during drafting or drop prematurely. In such cases, it would be useful for the draft host to be able to just abort the draft so that the draft can be restarted quickly and smoothly. The aborting would throw everyone out of the draft, preferably. Maybe even put the players into the new draft lobby automatically?

Currently every single player needs to leave the draft on their own in such a situation, and there's often difficulties communicating and getting everyone to notice the situation and to leave the ongoing draft to join the new one.