Enhancement: Added app banner for mobile devices

We've added an app banner at the top of the web view for mobile devices asking the user whether they want to continue working in the app. By dismissing it, it will not show again until a new session is started, e.g. by opening a new tab.

https://github.com/owncloud/web/pull/9696
https://github.com/owncloud/web/pull/9872

