Add email notification option ATTENTION_SET_ONLY

This new option was recently added to the backend in change 284133.

Change-Id: Ifef2aed071f646d4694fad48f8e7d47846ded8c3

polygerrit-ui/app/constants/constants.ts

diff --git a/polygerrit-ui/app/constants/constants.ts b/polygerrit-ui/app/constants/constants.ts
index 2db076f..be52835 100644
--- a/polygerrit-ui/app/constants/constants.ts
+++ b/polygerrit-ui/app/constants/constants.ts
@@ -292,6 +292,7 @@
 export enum EmailStrategy {
   ENABLED = 'ENABLED',
   CC_ON_OWN_COMMENTS = 'CC_ON_OWN_COMMENTS',
+  ATTENTION_SET_ONLY = 'ATTENTION_SET_ONLY',
   DISABLED = 'DISABLED',
 }