SPARK-1202: Improvements to task killing in the UI.

1. Adds a separate endpoint for the killing logic that is outside of a page.
2. Narrows the scope of the killingEnabled tracking.
3. Some style improvements.

Author: Patrick Wendell <pwendell@gmail.com>

Closes #386 from pwendell/kill-link and squashes the following commits:

8efe02b [Patrick Wendell] Improvements to task killing in the UI.
(cherry picked from commit 44f654eecd3c181f2aeaff3871acf7f00eacc6b9)

Signed-off-by: Patrick Wendell <pwendell@gmail.com>