Created and styled a header with navbar and search.

header.inc.php now actually outputs a header. It has links to the home page, notifications page, my profile page, edit profile page, and logout. It also has a search bar. The notifications page, view profile page, and search page have all yet to be implemented.

The header is only shown when the user is logged in. In order to have a way back to the login page at all times, any forms besides login that can be used without being logged in (register, send recovery email, recover password) have a "cancel" link next to the submit button. This cancel link brings the user back to the login page.