In pgauth plugin, get email address in message from settings

When generating an error message with an email address in it, get that
email address from the settings instead of being hardcoded. This is not
that important on the postgresql.org properties, but we have some
downstream uses of individual pieces that can benefit from it.

Author: Célestin Matte <celestin.matte@cmatte.me>

Author: mhagander

tools/communityauth/sample/django/auth.py

@@ -164,16 +164,21 @@ def auth_receive(request):
         # somehow fix that live, give a proper error message and
         # have somebody look at it manually.
         if User.objects.filter(email=data['e'][0]).exists():
+            if hasattr(settings, 'ADMINS') and len(settings.ADMINS) > 0:
+                contact = settings.ADMINS[0][1]
+            else:
+                contact = "webmaster@postgresql.org"
+
             return HttpResponse("""A user with email %s already exists, but with
 a different username than %s.
 
 This is almost certainly caused by some legacy data in our database.
-Please send an email to webmaster@postgresql.org, indicating the username
+Please send an email to %s, indicating the username
 and email address from above, and we'll manually merge the two accounts
 for you.
 
 We apologize for the inconvenience.
-""" % (data['e'][0], data['u'][0]), content_type='text/plain')
+""" % (data['e'][0], data['u'][0], contact), content_type='text/plain')
 
         if getattr(settings, 'PGAUTH_CREATEUSER_CALLBACK', None):
             res = getattr(settings, 'PGAUTH_CREATEUSER_CALLBACK')(