Issue https://github.com/OpenACalendar/OpenACalendar-Web-Core/issues/104 - remove link to Display Board.

This is terrible in v1.3.x. In v1.4.0 it will be in a seperate extension and will be better.
Issue https://github.com/OpenACalendar/OpenACalendar-Web-Core/issues/104 - move display board to seperate extension. Small core is good!

This should end up in v1.4.0.

This commit should introduce no functionality changes.