Enhancement: Redesign notifications

We have redesigned the notifications component to fit the overal new look of the web frontend, e.g. adding shadow and rounded corners.
It can now also be rendered "unpositioned" instead of having it always stick to the top of the screen.

https://github.com/owncloud/owncloud-design-system/pull/2210
https://github.com/owncloud/owncloud-design-system/pull/2216
https://github.com/owncloud/web/issues/7082
