As a minutes participant I want to optionally see only open topics of the current meeting minute to quickly spot to-be-discussed topics

• checkbox "Only open topics" behind the ExpandTopics button
• checkbox is initially "de-selected" on opening new minutes
• if checkbox is "true" only open topics are shown in the list
• if checkbox is "true" and a topic is marked "done" - the topic get invisible
• state is realized "per session"
• state is realized with a ReactiveVar