MM-11272 Added initial post metadata (#9175)

* MM-11272 Added app.PreparePostForClient

* MM-11272 Added app.PreparePostListForClient

* MM-11272 Added EmojiStore.GetMultipleByName

* MM-11272 Added emojis to PreparePostForClient

* MM-11272 Added unit tests for getting reaction counts

* MM-11272 Added unit tests for TestPreparePostForClient

* MM-11272 Added emojis from reactions to Post.Emojis

* MM-11272 Always update post.UpdateAt when reactions change to bust cache

* Fixed merge conflicts

* Moved post metadata-related code into its own file

* Update store mocks

* Fixed typo

* Add missing license headers

* Updated post metadata tests when custom emojis are disabled

* Fix unreliable unit tests

* Fix inconsistent casing in SQL statements

* Fix blank line

* Invalidate store cache after making changes

* Clear post cache synchronously with reactions

api4/channel.go

@@ -488,8 +488,13 @@ func getPinnedPosts(c *Context, w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	w.Header().Set(model.HEADER_ETAG_SERVER, posts.Etag())
-	w.Write([]byte(c.App.PostListWithProxyAddedToImageURLs(posts).ToJson()))
+	clientPostList, err := c.App.PreparePostListForClient(posts)
+	if err != nil {
+		mlog.Error("Failed to prepare posts for getFlaggedPostsForUser response", mlog.Any("err", err))
+	}
+
+	w.Header().Set(model.HEADER_ETAG_SERVER, clientPostList.Etag())
+	w.Write([]byte(clientPostList.ToJson()))
 }
 
 func getPublicChannelsForTeam(c *Context, w http.ResponseWriter, r *http.Request) {