Gives .rs-edit buttons 0 padding, like other icon buttons. Allow icon bu...

...ttons to provide hidden textual content to improve accessibility of icon buttons.

.rs-edit buttons are inheriting padding from user agent stylesheets because they do not override padding. this isn't a visible problem until you try to put text content inside a button, and notice that text-indent: 100% does not push text content out of view.
A mixin would clean this up.