Improve navigation

The main feature pages have now a navigation bar which links to other active features. If there is only one active feature the bar will not be displayed for participants. Creators always have at least two bar items, comments and live survey. For moderators the bar is used to switch between public and unreleased comment list. The navigation and the components itself looking different on mobile and desktop devices.

For desktop devices the sidebar, which is currently used for active user count in room overview, is used to navigate. For the implementation the old 'sidebar' has been refactored to use them in different use cases. As long as there is enough space other views (features) are not affected by this.

For mobile devices there is a bottom bar for the navigation according to material design guidlines. The layout of the participants feedback view has been reworked, so that the answer buttons and the chart is fully responsive to at least 500px. The comment view detects if the navigation bar is active to decide on which height the bottom buttons will be displayed.

Related: #665 (closed)