Implement submission contributors and roles interface

[pkalita-lbl]

The submission portal backend has the ability to attach additional contributors to a submission and to assign them roles, and the submission portal frontend has a UI that looks like this:

A number of beta testers expressed interested in allowing multiple people to work on a submission together. The contributors/roles mechanism would be the current way to do that. But the app doesn't include any UI support for it yet.

This task is to:

1. Design what that UI would look like in the app
2. Implement the UI to add/remove contributors for a submission, as well as setting their role/permission level.
3. Ensure that the existing app interfaces respect the user role levels (e.g. a contributor with the Viewer role should not be able to edit anything in that submission)