Navbar hide/show tweaks

Fixes #2563.

Summary of changes:
* Updates styling and text of sidebar button.
* Makes sidebar disappear when clicking outside.

@jamalex please review.
Yes, clever me. Who added that method anyway?!

My concern with that approach is that it effectively disables content while the sidebar is out. If that is the case, I would want to grey out the background layer slightly to indicate that stuff obscured is non-functional.