refined _get_user_flagger()
refined _get_user_flagger()

Had a use case where COMMENT_FLAG_USER_ID was not set, and 'Zinnia-Flagger' user had already been created.  This fix prevents a the following error:

IntegrityError: duplicate key value violates unique constraint "auth_user_username_key"
DETAIL:  Key (username)=(Zinnia-Flagger) already exists.
aorzh/django-blog-zinnia,aorzh/django-blog-zinnia,Fantomas42/django-blog-zinnia,ghachey/django-blog-zinnia,Maplecroft/django-blog-zinnia,Fantomas42/django-blog-zinnia,Zopieux/django-blog-zinnia,dapeng0802/django-blog-zinnia,petecummings/django-blog-zinnia,dapeng0802/django-blog-zinnia,Maplecroft/django-blog-zinnia,1844144/django-blog-zinnia,extertioner/django-blog-zinnia,extertioner/django-blog-zinnia,Zopieux/django-blog-zinnia,bywbilly/django-blog-zinnia,marctc/django-blog-zinnia,Fantomas42/django-blog-zinnia,ZuluPro/django-blog-zinnia,1844144/django-blog-zinnia,ghachey/django-blog-zinnia,petecummings/django-blog-zinnia,ZuluPro/django-blog-zinnia,dapeng0802/django-blog-zinnia,marctc/django-blog-zinnia,Zopieux/django-blog-zinnia,Maplecroft/django-blog-zinnia,ghachey/django-blog-zinnia,ZuluPro/django-blog-zinnia,aorzh/django-blog-zinnia,bywbilly/django-blog-zinnia,marctc/django-blog-zinnia,extertioner/django-blog-zinnia,1844144/django-blog-zinnia,petecummings/django-blog-zinnia,bywbilly/django-blog-zinnia