Editing channel start- or end vertices should disconnect channel from connection node

[leendertvanwolfswinkel]

If a user

• Moves the first or last vertex of a channel
• Adds an extra vertex to a channel at the start or end

There are two possibilities:

1. The start or end vertex' new position is on a connection node
2. The new start or end vertex is not on a connection node

In case 1, the channel's connection_node_start_id or connection_node_end_id should be updated to the id of that connection node

In case 2, the channel's connection_node_start_id or connection_node_end_id should be emptied