Make OAuth client id accessible to new ui w/o hardcoding

appengine/swarming/proto/config.proto

 // Copyright 2016 The LUCI Authors. All rights reserved.
 // Use of this source code is governed under the Apache License, Version 2.0
 // that can be found in the LICENSE file.
 
 syntax = "proto2";
 
 
 // Schema for settings.cfg service config file in luci-config.
 message SettingsCfg {
   // id to inject into pages if applicable.
@@ skipping 13 matching lines @@
 
   // Configuration for swarming-cipd integration.
   optional CipdSettings cipd = 6;
 
   // Configuration for swarming-mp integration.
   optional MachineProviderSettings mp = 7;
 
   // Emergency setting to disable bot task reaping. When set, all bots are
   // always put to sleep and are never granted task.
   optional bool force_bots_to_sleep_and_not_run_task = 8;
+
+  optional string ui_client_id = 9;

[Vadim Sh., 2016/08/11 20:00:24]

add a comment explaining what it is and (briefly) how to get it

 }
 
 
 // Configuration for swarming-isolate integration.
 message IsolateSettings {
   // URL of the default isolate server to use if it is not specified in a
   // task. Must start with "https://" or "http://",
   // e.g. "https://isolateserver.appspot.com"
   optional string default_server = 1;
 
@@ skipping 26 matching lines @@
   // task.
   optional CipdPackage default_client_package = 2;
 }
 
 
 // Settings for Swarming-MP integration.
 message MachineProviderSettings {
   // Whether or not Swarming-MP integration is enabled.
   optional bool enabled = 1;
 }