Permit multiple owners of a note

It will be very usefull to let a note have multiple owners. So we all could change the read and write permissions, or any other config for one particular note.

Context : we are a small team with our own codimd instance, so it's not really the same as in #12, no need to have multiple groups in the same instance (but if it's implemented, we could have one only group in our instance, why not). When someone in the team is not here, and it's him/her who created a note, the others must be able to change the note config, that's the important need.

In the config panel of a note, a selector should let us add other owners.