Give all platform blocks in submission page equal height (bug 964961)

https://bugzilla.mozilla.org/show_bug.cgi?id=964961

Tested with IE9 and more recent browsers. I don't have Safari though.

Screenshots with Firefox:

![normal](https://cloud.githubusercontent.com/assets/187006/4337018/b66c353a-400b-11e4-89f5-ecf0275c3f8d.png)
![with custom error message added](https://cloud.githubusercontent.com/assets/187006/4337043/dba02e60-400b-11e4-914c-8797a359aa74.png)




I think there's a `display-flex();` helper to add the prefixes for this.