Various improvements

• refine "demo-session"-button in room-joining
• refine content-groups-list
• the app should always shows the top if its overloaded
• the footer should contain an imprint, privacy policy and blog
• content-edit title ellipse
• add "no answers"-status to statistic-list-info-dialog
• show the abstentions in the diagram only if any exist
• fix card-width in statistic-list component
• fix icon-button-bugs