Move LDAP sync time constant to a method

This moves the `1.hour` constant used as the LDAP sync time to a
separate method. This allows EE to override this method using a module,
instead of directly modifying the source code.

app/models/user.rb

@@ -941,12 +941,17 @@ class User < ActiveRecord::Base
     if !Gitlab.config.ldap.enabled
       false
     elsif ldap_user?
-      !last_credential_check_at || (last_credential_check_at + 1.hour) < Time.now
+      !last_credential_check_at || (last_credential_check_at + ldap_sync_time) < Time.now
     else
       false
     end
   end
 
+  def ldap_sync_time
+    # This number resides in this method so it can be redefined in EE.
+    1.hour
+  end
+
   def try_obtain_ldap_lease
     # After obtaining this lease LDAP checks will be blocked for 600 seconds
     # (10 minutes) for this user.