Use common form style for homepage and mobile page.

Add mixin for input box, use mixin for button on homepage.

@dtothefp Can you review?
@stewsmith hey sorry, the L/R padding should be 4px.  The reason we weren't seeing this is the anim-icon CSS, but it could be an issue in the future.  May have to update the width of your input if you adjust this.  After that LGTM