`keepViewportAtBoom` directive has been renamed to `followsAppendedContent` + does not autoscroll to bottom anymore while user is checking previous content (i.e. scrolled up manually).

When user manually returns to the bottom, the directive works normally again from then on.
Setting ShoutboxWebcomponentCtrl's `forceScrolling` to the directive's $scope (via directive's `forceFollowIf` attribute) ensures autoscrolling in case the user itself writes a new content.
Moves Firebase Shouts data access into a DAO, which uses a new configurable Firebase Datasource