Bug 1101760 - Clear classification fully on an unpin all

This work fixes Bugzilla bug [1101760](https://bugzilla.mozilla.org/show_bug.cgi?id=1101760).

Not a huge bug fix here, but this clears both the classification comments field, and resets the value for the classification drop down menu, when the user Clears the pinboard (formerly 'Un-pin all').

I've renamed the button in the menu cluster, so that it matches. I needed to add a 3px compensation unfortunately, so it looks sort-of justified with the save button text below it. Here's the label change:

![clearallpinboardcurrent](https://cloud.githubusercontent.com/assets/3660661/5483704/88497818-8645-11e4-84ff-7f8f1ef95ef5.jpg)

![clearallpinboardproposed](https://cloud.githubusercontent.com/assets/3660661/5483705/8b686e82-8645-11e4-8cbd-9f1477095539.jpg)

Unless someone feels strongly about it, I don't think it's worth changing all the method names from `.unPinAll` to `.clearAll` etc, since the UI still may be a bit of a moving target, and could change again.

I imagine I'll be able to remove that css tweak later when hopefullly we migrate these buttons underneath the upcoming one-line classification comment field, still to be landed.

Tested on Windows:
FF Release **34.0**
Chrome Latest Release **39.0.2171.95 m**

Adding @camd  for review, and @edmorley for visibility.