Provide supplementary close icon for pinboard (1045099)

This work fixes Bugzilla bug [1045099](https://bugzilla.mozilla.org/show_bug.cgi?id=1045099).

As requested this adds a supplementary close icon into the top right hand corner of the pin board to hide it, in addition to its existing expand/collapse (aka. show/hide) chevron visible underneath. Unlike adjacent containers I just created a discrete div wrapper for the header, rather than wrapping the other UI contents below it within that div.

I was going to try to leverage `navbar-nav` to utilize the existing grey anchor color, but that exists only within top level `li` elements - which this is not. So I just created its own css entry for this header to assign its color. Maybe there is a better way.

Here is the appearance after the fix:

![supplementarypinboardclose](https://cloud.githubusercontent.com/assets/3660661/3770706/ce18a1ae-18ed-11e4-950d-3bd0be0599c2.jpg)

Tested on Windows:
FF Release **31.0**
Chrome Latest Release **36.0.1985.125 m**

Adding @jeads and @camd for visibility.