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

app/post_test.go

@@ -466,7 +466,6 @@ func TestImageProxy(t *testing.T) {
 			list := model.NewPostList()
 			list.Posts[post.Id] = post
 
-			assert.Equal(t, "![foo]("+tc.ProxiedImageURL+")", th.App.PostListWithProxyAddedToImageURLs(list).Posts[post.Id].Message)
 			assert.Equal(t, "![foo]("+tc.ProxiedImageURL+")", th.App.PostWithProxyAddedToImageURLs(post).Message)
 
 			assert.Equal(t, "![foo]("+tc.ImageURL+")", th.App.PostWithProxyRemovedFromImageURLs(post).Message)