Bug 1079650 - Give the Initials buttons consistent size and spacing

This fixes Bugzilla bug [1079650](https://bugzilla.mozilla.org/show_bug.cgi?id=1079650).

Nothing dramatic here, we just tweak the Initials buttons in the revision list columns, to be a consistent size and spacing, so both their appearance, and the negative space around them is no longer ragged.

Here is the before and after:

![initiallabelscurrent](https://cloud.githubusercontent.com/assets/3660661/4553756/f1c22390-4e9f-11e4-91a1-64541a5c43fe.jpg)

![initiallabelsproposed](https://cloud.githubusercontent.com/assets/3660661/4553758/fe98676e-4e9f-11e4-9cda-86853b9d2a3a.jpg)

Everything appears to be fine in both Firefox and Chrome, with both the regular repo view, and individual resultset view. I did a lot of blink-testing to ensure the subtle change consumes no more additional line width. Let me know if this looks correct for you guys on your various platforms.

There didn't seem to be any flexbox approach required in this particular case.

Tested on Windows:
FF Release **32.0.3**
Chrome Latest Release **37.0.2062.124 m**

Adding @wlach and @camd for review.