Plan Collaborators With "Read only" Permissions Are Still Able to Add Contributors to a Plan

[aaronskiba]

Please complete the following fields as applicable:

What version of the DMPRoadmap code are you running? (e.g. v2.2.0)
4.1.1+portage-4.1.1 4.1.1+portage-4.1.0

Expected behaviour:

• I'm not 100% sure. However, I'd assume that collaborators with "read only" access on a plan shouldn't be able to modify it any way.

Actual behaviour:

• Plan collaborators with "read only" can successfully add a contributor to a plan.
  Steps to reproduce:
• 1. Add a collaborator and give them "read only" access
• 
• 2. Sign into the app as the added "read only" user and navigate to the corresponding plan
• 
• 3. Try adding a contributor to the plan
•