[DOP-28389] Improve group member role check

[dolfinus]

Change Summary

If user has superuser permission, GET /v1/groups returns role: SUPERUSER, but GET /v1/groups/:id doesn't. Applied a fix, and also simplified permission check a bit.

Related issue number

Checklist

• Commit message and PR title is comprehensive
• Keep the change as small as possible
• Unit and integration tests for the changes exist
• Tests pass on CI and coverage does not decrease
• Documentation reflects the changes where applicable
• docs/changelog/next_release/<pull request or issue id>.<change type>.rst file added describing change
  (see CONTRIBUTING.rst for details.)
• My PR is ready to review.

[github-actions]

Coverage Report •

File	Stmts	Miss	Branch	BrPart	Cover	Missing
syncmaster/db/repositories
group.py	143	10	36	5	93%	162–163, 199, 238–239, 366–369, 377
queue.py	83	5	24	1	93%	70–71, 79–80, 179
TOTAL	5354	637	828	171	88%