[ticket/12209] Notify the admin that OAuth still allows DB authentification

PHPBB3-12209
Merge pull request #2583 from nickvergessen/ticket/12209

[ticket/12209] Notify the admin that OAuth still allows DB authentification
Merge branch 'develop-ascraeus' into develop

# By Joas Schilling
# Via Dhruv Goel (1) and Joas Schilling (1)
* develop-ascraeus:
  [ticket/12209] Notify the admin that OAuth still allows DB authentification