[SPARK-4463] Add (de)select all button for add'l metrics.

This commit removes the behavior where when a user clicks
"Show additional metrics" on the stage page, all of the additional
metrics are automatically selected; now, collapsing and expanding
the additional metrics has no effect on which options are selected.
Instead, there's a "(De)select All" box at the top; checking this box
checks all additional metrics (and similarly, unchecking it unchecks
all additional metrics).

This commit is intended to be backported to 1.2, so that the additional
metrics behavior is not confusing to users.

Now when a user clicks the "Show additional metrics" menu, this is what
it looks like:
![image](https://cloud.githubusercontent.com/assets/1108612/5094347/1541ead6-6f15-11e4-8e8c-25a65ddbdfb2.png)

Author: Kay Ousterhout <kayousterhout@gmail.com>

Closes #3331 from kayousterhout/SPARK-4463 and squashes the following commits:

9e17cea [Kay Ousterhout] Added italics
b731230 [Kay Ousterhout] [SPARK-4463] Add (de)select all button for add'l metrics.