Enhancement: Add sign-out icon

There has been confusion in user experience about the 
current usage of the `exit_to_app` icon as sign-out icon.
We have added a dedicated `sign-out` icon to avoid this 
in the future.

https://github.com/owncloud/web/issues/5590
https://github.com/owncloud/owncloud-design-system/pull/1551
