Bug 1112238 - Create a UI help link to better access the build version

This fixes Bugzilla bug [1112238](https://bugzilla.mozilla.org/show_bug.cgi?id=1112238).

Nothing exciting, but it seems it would be helpful to access the build revision via the UI.

I had to ask in channel where the file was and I am pretty sure I might not be the only one, or I might later forget the location and have to ask again :)

This puts a nice subtle Build link in a new footer at the bottom right of the help page. It points to where I understand the `media/revision` file to be.

![helpbuildlinkproposed](https://cloud.githubusercontent.com/assets/3660661/5466882/c7c057b4-8582-11e4-95fa-60ec844fe763.jpg)

I've simulated that location locally and assuming I have the correct path, it seems to be working fine.

I added a generic `midgrey` class and used the same targets as `lightgrey`, in case we want to leverage it elsewhere for other elements. The help markup is a bit inconsistent but I tried to match its indentation logic.

I assume we may also want to turn off caching for the revision file, so when the user loads the file it will always be current.

Tested on Windows:
FF Release **34.0**
Chrome Latest Release **39.0.2171.95 m**

Adding @maurodoglio  for review, and @edmorley for visibility.