Add Rubocop to prevent new usages of Default Organization

Some new usages of Organizations::Organization.default_organization showed up.

We do have a cop that flags usages of the "default" trait on Organization factory. This cop does not detect create(:organization, id: Organizations::Organization::DEFAULT_ORGANIZATION_ID).

And we also don't detect use of Organizations::Organization.default_organization

(https://docs.gitlab.com/development/organization/#the-default-organization)

Proposal

• Update docs: We need to highlight that self-managed also doesn't have a guarantee that the Default Organization exist.
• Modify AvoidCreateDefaultOrganization cop:
  • Add a link to the MSG: https://docs.gitlab.com/development/organization/#the-default-organization
• Create new cop AvoidDefaultOrganization:
  • This should detect usages of Organizations::Organization.default_organization
  • Include a link to https://docs.gitlab.com/development/organization/#the-default-organization
• Create new cop AvoidConstDefaultOrganizationId
  • This should detect the usage of DEFAULT_ORGANIZATION_ID in specs or application code
  • Include a link to https://docs.gitlab.com/development/organization/#the-default-organization

Next step would be to refactor the new usages of Organizations::Organization.default_organization. We need to coordinate with the team(s) that are owner of the code that is now using the default organization.