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

model/emoji.go

@@ -7,6 +7,7 @@ import (
 	"encoding/json"
 	"io"
 	"net/http"
+	"regexp"
 )
 
 const (
@@ -14,6 +15,8 @@ const (
 	EMOJI_SORT_BY_NAME    = "name"
 )
 
+var EMOJI_PATTERN = regexp.MustCompile(`:[a-zA-Z0-9_-]+:`)
+
 type Emoji struct {
 	Id        string `json:"id"`
 	CreateAt  int64  `json:"create_at"`