Do not inject a #app-navigation-toggle if there is already one

This allows apps to ship their own, as in some cases the #app-content
element does not exist on page load and therefore the injection fails
and the icon is missing afterwards.

Fixes https://github.com/nextcloud/server/issues/14956



Signed-off-by: Christoph Wurst <christoph@winzerhof-wurst.at>