[DOP-29772] Change Keycloak redirect from 307 to 401

[dolfinus]

Change Summary

Browser redirects on local Keycloak installation are broken because of CORS. Instead return 401 code which will be handled by UI.

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	Cover	Missing
syncmaster/server
handler.py	157	25	84%	57, 62, 70, 72, 76, 81, 91, 112, 118–120, 210–212, 226–227, 241–242, 315–317, 329–332
syncmaster/server/api/v1
auth.py	21	2	90%	32, 40
syncmaster/server/providers/auth
keycloak_provider.py	75	9	88%	26–28, 71–72, 105–107, 110
TOTAL	4747	263	94%