Convert Project/Org relationship to an M2M

[ghickman]

We expected each project to be owned by a single organisation when we added both objects, however in reality it's possible for two or more orgs to collaborate on a project.

We can model this by converting the existing ForeignKey to a ManyToManyField.

We will also need to remove orgs from the main URL structure. This should be trivial to manage by setting up the apppriate redirect instances on all projects. (done in #3312)

We also need to handle various staff area controls switching to a multiple item selection UI.

[madwort]

@bloodearnest to review this week