Bug 1113843 - Draw conditional resultset btns correctly on all browsers

This fixes Bugzilla bug [1113843](https://bugzilla.mozilla.org/show_bug.cgi?id=1113843).

For sheriffs or users in the Try repo where the Cancel All Jobs button is displayed, we weren't displaying properly on Chrome.

This change fixes that, and provides a consistent spacing between all the buttons, for all browsers. @wlach's earlier flexbox effects are still present and ensure these buttons flow at small browser widths.

Here's the before:

![conditionalresultsetbtnscurrent](https://cloud.githubusercontent.com/assets/3660661/5540833/464bfd52-8a9e-11e4-92d7-95753fed17fa.jpg)

Here's the after:

![conditionalresultsetbtnsproposed](https://cloud.githubusercontent.com/assets/3660661/5540835/509fe570-8a9e-11e4-9c01-c8a5e01b1066.jpg)

We also now collapse nicely when the Cancel button is suppressed (eg. mozilla-central, regular user):

![conditionalresultsetbtnscenral](https://cloud.githubusercontent.com/assets/3660661/5540838/6ababa02-8a9e-11e4-848e-038b6144d441.jpg)

It appeared we need to leave that orphan `span` in place on line 20, so the child elements receive their correct flex spacing. Without it the flex webkit margins are lost on the first buttons.

Tested on Windows:
FF Release **34.0**
Chrome Latest Release **39.0.2171.95 m**

Adding @camd for review and @edmorley for visibility.