Backport of #12054 to stable13

only write when the displayname differs, but then announce it

refs #5212 and fixes #9112

Signed-off-by: Arthur Schiwon <blizzz@arthur-schiwon.de>

do not run into UniqueConstraintViolationException

… when an unmapped user logs in for the first time when background job
mode is ajax and no memcache was configured.

Signed-off-by: Arthur Schiwon <blizzz@arthur-schiwon.de>