Clean up contact model in DB

[jplitza]

Currently, multiple contact possibilities are stored as comma-separated string in the database. The cleaner solution would obviously be to have a separate table contacts and a joining table node_contacts. That could give the source plugins the responsibility to split their format into multiple contacts.

[rubo77]

This would be great, because we want to be able to add unlimited extra external contacts to each node.

Any users, that wants to be notified on a node, should be able to opt-in on any node.

I will create an external applicat where other users can add their notification requests.