Add option to 'Join notebook' for admins

Currently admins in an account can 'take over' any notebook they are not a collaborator on. When this action is taken, the user is made the owner of the notebook (which doesn't appear to affect anything) and then is returned back to the Notebooks dashboard where they then have to find the notebook again to edit it.

It is often the case that we need to collaborate on each others' notebooks, and to make things easier, I am suggesting we add a 'Join notebook' option to the menu. Clicking this button would add the user as a collaborator on the notebook and then take them in to view the notebook directly.