Fix closing menu on second click in sharing page

When a "mouseup" event was triggered on any element except on the share
menu or its descendants the share menu was closed. The share menu toggle
is not a descendant of the share menu, so clicking on it when the share
menu was shown closed it by removing its "open" CSS class. However, once
that happened the click event was then handled by the share menu toggle,
which toggled the "open" CSS class in the share menu and thus added it
again. So, from the user point of view, nothing happened when clicking
on the share menu toggle if the share menu was open.

Now a "mouseup" event on the share menu toggle no longer closes the
share menu, and thus toggling the "open" CSS class when handling the
"click" event works as expected.

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