Search style updates

I've heard from many folks that the search input was confusing, that it could be mistaken for an input for the login label that precedes it.

I've moved the search icon to the left side of the input to visually separate it from the login label to its left. The button wasn't necessary because as you type, the search returns results on the fly.

I added "Search" placeholder text.

I removed the animated input width (which could appear jarring on Chrome).

The `wdn-input-group` markup (it keeps the button and the input together) is no longer needed and can be removed along with the associated CSS.