shell: Simplify HTML

Mostly, move the top navbar out of #content.
shell: Basic, ugly, host switcher.

The HTML is all wrong.
shell: Hide navigation when showing the dashboard.
shell: Position the sidebar explicitly and correctly.

Also, specify the height of images in the navbar explicitly so that it
doesn't change size asynchronously when they load.
shell: Clean up layout recalculation a bit.

Now we resize iframes when updating the navigation elements, since it
always needs to happen at the same time anyway and in the right order.

Additionally, the layout is recalculated after every change of the navbar.
shell: Style the sidebar nicely.

From Andreas Nilsson.
shell: Highlight selected host in the switcher.
shell: Show avatars in host switcher.