Unable to unassign/modify user invited as Owner

[laurodias]

What happened:
When a user is invited to a project as Owner, it can't be unassigned or have its permissions changed. It's also not possible to leave the project from the user's setting page.

The behaviour is the same if we try managing local or external OAuth2 users.

What you expected to happen:
Admin user and/or project owners should be able to manage any members assigned to a project, including removing and changing permissions.

How to reproduce it (as minimally and precisely as possible):

• Login as Litmus admin user
• Create a project
• Create a user
• Invite the user to the project as Owner
• Login as user and accept invitation
• Login back as admin and try to change user's permissions or remove it from the project