OIDC: Reference group by FK, set workspace attribute to False

[psrok1]

Your checklist for this pull request

• I've read the contributing guideline.
• I've tested my changes by building and running the project, and testing changed functionality (if applicable)
• I've added automated tests for my change (if applicable, optional)
• I've updated documentation to reflect my change (if applicable)

What is the current behaviour?

• Group created for OIDC provider is referenced by name but should be referenced by id
• Workspace attribute is set to True which cannot be changed and causes "automatic" sharing of uploads between users coming from the same provider

What is the new behaviour?

• OIDC group is referenced by foreign key
• Set workspace attribute to False for all OIDC groups

Test plan

Tested manually: both migration and fresh creation of the OIDC provider and members.

Closing issues

closes #973