Editor for accounts/groups list.

To implement some new UI feature, we need a way to specify list
of accounts and/or groups. Similar solutions already exists for 'change'
view, but it can't be reused as-is in other places.
In this commit the existing solution was reworked to use the same
editor in different places.
Editor was splitted to 2 parts - editor itself and suggestion provider.

Change-Id: I43ce060e568a69f9842fbfad6f5fd62361ab2022

polygerrit-ui/app/template_test_srcs/template_test.js

@@ -38,6 +38,8 @@ const EXTERN_NAMES = [
   'SiteBasedCache',
   'FetchPromisesCache',
   'GrRestApiHelper',
+  'GrDisplayNameUtils',
+  'GrReviewerSuggestionsProvider',
   'moment',
   'page',
   'util',