Hide mobile nav dropdowns

Fixes #447.

"rendered" only runs on initial render, so when a user was signed out and then signed in, the signed in menus were rendered (e.g. admin, notifications) the "rendered" function wasn't called so then the menus weren't hidden, but the menus that had been rendered signed out (e.g. view) were still shown collapsed. Doing this in CSS simplifies everything and makes sure the menus are consistently hidden until the user shows them.

Also, take a look at .search-clear. As far as I can tell it's completely unused, so I just deleted it.