Show real username on mouseover with usernameHider enabled

It'd probably be better to do this with JavaScript instead of ugly CSS hacks, but this will do for now. Fixes #1139.
corollary to https://github.com/honestbleeps/Reddit-Enhancement-Suite/pull/1698/files#r19376570 -- we could preserve the original .textContent with `$(...).data('originalTextContent')` like in localDate.