[ticket/12562] Add max-width to proSilver

Don't kill me just yet. There are (many) valid reasons to have a max-width. Legibility, style consistency, and a default for extensions. I'm obviously not saying that A51 should use it, but having a default instead of "unlimited" for releases is worth considering.

I'm just submitting this so we know what we're looking at.
![fx](https://cloud.githubusercontent.com/assets/3015760/3237328/23044a26-f0e7-11e3-83f6-801c07163939.png)

Discussion: https://tracker.phpbb.com/browse/PHPBB3-12562