Don't suggest service users as reviewers

Service users usually decide based on their own criteria which changes
they are acting on and don't react on review invitations sent by email.
Add a new setting suggest.skipServiceUsers to gate this behavior.

Bug: Issue 14117
Change-Id: If0202fffda8ce173f2317d61c4f5387e18868736

Documentation/config-gerrit.txt

diff --git a/Documentation/config-gerrit.txt b/Documentation/config-gerrit.txt
index c362710..9d7e93b 100644
--- a/Documentation/config-gerrit.txt
+++ b/Documentation/config-gerrit.txt
@@ -5140,6 +5140,13 @@
 +
 By default 50.
 
+[[suggest.skipServiceUsers]]suggest.skipServiceUsers::
++
+If link:access-control.html#service_users[service users] should be skipped when
+suggesting reviewers.
++
+By default true.
+
 [[tracing]]
 === Section tracing