Add a 'kill' executor link in the Web UI.

The executor which is running in a bad node(eg. The system is overloaded or disks are busy) or has big GC overheads may affect the efficiency of job execution, although there are speculative mechanisms to resolve this problem, but sometimes the speculated task may also run in a bad executor.
We should have a 'kill' link for each executor, similar to what we have for each stage, so it's easier for users to kill executors in the UI.