Add page title to header bar

Title determined with CustomTitleStrategy is reused to show the current page title on left side of the header

For all pages outside a room, always just show the current page title. In a room, always show the room name. For room suboutes show chevron icon and the title next to room name.

On mobile devices, always just show the current title. To prevent from overflow use the TextOverflowClipComponent. In a room, limit the room name width first if the subtitle is too long.

The back button has been removed since hierarchy routing will no longer be needed in the future.

NOTE: Currently, there are two routes, which still need back navigation: templates/{ID} and p/series/{NAME}/{INDEX}. For both browser back navigation can be used for now.