Implement live graph sharing

[cwerling]

Based on a longer discussion in the team and with the potential users, here are the requirements:

• Graphs can be shared by multiple people, but only one of them can lock the graph for editing.
• Everybody else sees a regularly updated read-only version of the graph in the editor.
• This demands some UI extension for user sharing management.
• The updates are sent from the web server to the JS code vis WebSockets, in the moment were it receives the frontend API calls from the editing user.
• The update messages are formulated in a way that most JS code can be re-used (command pattern implementation.)

This should be an extension of the meanwhile existing static graph sharing UI.