Various header clean ups.

1. Removes the sign in form from the homepage header, replacing it with buttons to either register or sign up. Creates an extra step for the user, but de-clutters the homepage.
1. Cleans up the header search input so that its width is manually set for all viewports. It goes to a second row on tablets and phones and is as wide as possible on wide screens.
1. Tweaks bootstrap to make the header taller.
1. Makes the image in the non-homepage header 8px wider to create better horizontal rhythm. With this change the A from About lines up with the R from Result count, below.
1. Adds more links to the header for people that are signed in.