Fix error notification from `ApplianceStatus` service
Show flashing disconnection icon if we don't get response from backend

(or if we get an error from the backend).

The glyphicons font (used for the "disconnected" icon --"flash" is the
closest I could find to disconnected) comes with Bootstrap. The
Bootstrap css has the font path set to `../fonts/`, hence the symlink to
`vendor/fonts`.

The blinking is done using css animations. It didn't have a noticeable
impact on Firefox's cpu usage according to `top`. The blinking is done
by changing the colour to match the background colour (instead of
setting `visibility: hidden`) so that the `title` tooltip stays up if
you put your cursor over the blinking icon.
Remove unused dependency (unused since fcc50f03)