Missing websocket connection blocks some users

In some cases websocket connections get blocked.

For example over Marcs mobile connection websocket does only work with https:// connections, but not with http://  connections.

A Korean translator reported not working websocket connection.

Therefore we need to do the following:

• For the multi-user feature:
  • if a user without working websocket wants to open a task let him only open the task, if no one else works on it. In this case, block the task for all other users. Inform all users what happened and why
• For all other features, where websocket is used:
  • Poll the server side every 10 seconds for the needed status information