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.
@rtibbles Would be good to use `listenToDOM` or something, right? Otherwise, we could leave a zombie event handler.

Also, this event won't get triggered if a button etc is clicked that does a `return false` or similar, since that will prevent it bubbling up to `document`. I would suggest a transparent overlay with z-index just lower than the sidebar, but higher than the page elements, to intercept any click that happens.