Combining the Contributors/Collaborators feature

[mariapraetzellis]

The current Collaborators tab layout is confusing as it has two places to list people affiliated with the DMP. We should combine the listing so that you can easily list all contributors & collaborators in one place and simply click to invite a contributor to collaborate on the plan. We will need to define the roles more fully in order to accommodate sharing the plan with someone who is not a project collaborator.

We will further flesh out details on this merging of features, just adding the issue in now so we can plan for it in coming sprints.

[pherterich]

Also see #2823 #3040