Introduce Hashtag bulk action flow

Currently it is identical to the topic flow. Next I will make
modifications unique to the hashtag flow.

https://imgur.com/a/5LMMFNd

Google-Bug-Id: b/220852728
Release-Notes: skip
Change-Id: I2c8358d1eb0637e515d53875d8fa6f3b2d00fbbe
diff --git a/polygerrit-ui/app/test/mocks/gr-rest-api_mock.ts b/polygerrit-ui/app/test/mocks/gr-rest-api_mock.ts
index d91b438..ae8545a 100644
--- a/polygerrit-ui/app/test/mocks/gr-rest-api_mock.ts
+++ b/polygerrit-ui/app/test/mocks/gr-rest-api_mock.ts
@@ -280,6 +280,9 @@
   getChangesWithSimilarTopic(): Promise<ChangeInfo[] | undefined> {
     return Promise.resolve([]);
   },
+  getChangesWithSimilarHashtag(): Promise<ChangeInfo[] | undefined> {
+    return Promise.resolve([]);
+  },
   getConfig(): Promise<ServerInfo | undefined> {
     return Promise.resolve(createServerInfo());
   },