Fix collapse button in app navigation in IE11

Fix collapse button in app navigation in IE11

Although the collapse button has an absolute position Internet Explorer
11 needs the left position of that button to be explicitly set to 0
(other browsers do it implicitly due to the absolute position);
otherwise the button appears to the right of the link (and behind the
app content).

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>