Create AssignmentModel

[grassick]

• [x] Create a class to handle logic of determining who can edit/see assignments.

All admins of a form can admin assignments for the form. Managers of a deployment can admin assignments for their particular deployment. assignedTo people can view assignments. No one else.

• [x] Create a fixRoles function similar to the one in ResponseModel that computes all this.
• [ ] Enforce that assignments can't be created on server for a deployment that I'm not manager of if I am not an admin of the form
• [x] Ensure that server updates roles before saving to database
• [x] Add logic in FormCollectionModel to update roles of all assignments if roles of form are updated. There is already similar logic for responses there.

[mbriau]

so the assignement table should also have a roles column? (Not there yet)