Extend "bind this sub" functionality to leaders

[Wes Chao]

I didn't implement this before, since a bound leader disables the leader skill, and there was no good way to handle that in the old UI. The new UI allows us to just set the leader skill override to 1.

[Wes Chao]

I think this should be easy so I'll just go ahead and knock it out.

[Wes Chao]

Implemented; will send a pull request.