Rem to px

Switching to use `px` as non-relative sizing unit, since we've been having a lot of trouble getting rem polyfill to work over our new CDN due to cross-domain issues. It's not ideal, since it means we'll have more chances for slight pixel inconsistencies to go unnoticed but this probably helps us sync with design team better.

References DoSomething/dosomething#2678 and DoSomething/dosomething#2648.