Improve auto-scroll feature

[gvd1]

At the moment users with the appropriate Pause opeka chat auto-scroll permissions can choose to stop the auto-scroll by selecting the "Pause auto-scroll" checkbox in the chat view and in that way read messages uninterrupted.

We would like to improve the scrolling feature generally for all users in a way that resembles e.g. the way that Facebook Messenger works. That means

• when the user scrolls up to read old messages, the auto-scroll feature should be paused momentarily.
• If a new message arrives while the auto-scroll feature is paused, a little popup notification at the bottom of the message area should inform the user that "user x posted a message".
• when the user scrolls to the bottom again, the auto-scroll feature should kick back in.
• the "Pause auto-scroll" checkbox and the Drupal permissions are probably no longer necessary and should be deleted.