Allow users to hide the site header if they want to

Some users may want to automatically hide the site header/footer
assets upon sign-in, as they might take up a lot of screen space.

For example, the site header at http://review.source.android.com/
is 86 pixels tall.

Bug: GERRIT-45
Signed-off-by: Shawn O. Pearce <sop@google.com>