Support multiple project owners

[ludovic-pourrat]

In order to better manage Litmus, we would need to extend the current behaviour by allowing multiple projects owners. This is especially useful when integrating Litmus within SSO (single sign-on via OIDC) and where we need someone within the organisation to became the project owner in addition to the administrator created by default.

The purposed feature implementation can be described as follow.

Any created users can be invited into a project with the owner role. Then the default administrator will remain but not being used (not mapped to any user in the organisation).

Feedback appreciated.

A PR is attached to reflect this feature proposal.