Disallow bounties on certain tickets.

We found out that people tend to put bounties on our internal TODO lists, because they are written less technical. https://github.com/OpenRA/OpenRA/labels/Meta The problem is those bounties can often never be claimed, because they were not done by a single person and they are often long running goals spanning across a large amount of pull requests and patches. If we could define a label NOBOUNTY where no one could pledge, this might help steer things back into direction.
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