feat(oauth): Add option to logout from OpenID Connect provider

* Fixes #598

Signed-off-by: Marvin A. Ruder <signed@mruder.dev>

backend/prisma/schema.prisma

@@ -40,6 +40,8 @@ model RefreshToken {
 
   userId String
   user   User   @relation(fields: [userId], references: [id], onDelete: Cascade)
+
+  oauthIDToken  String? // prefixed with the ID of the issuing OAuth provider, separated by a colon
 }
 
 model LoginToken {