[MM-55143] Disallow reacting with an emoji that does not exist, limit the total number of unique reactions per post (#25331)

* [MM-55143] Disallow reacting with an emoji that does not exist

* WIP for server limit on emoji reactions

* WIP

* Implement default limit of 25 unique emoji reactions

* Add modal for reaction limit

* Fix test

* PR feedback

* Fix i18n

* Update admin string

* Merge'd

* Fixing some issues, check limits correctly based on other users reactions

* Fix typos

* Fix lint/test

* Add tests, fix other tests

---------

Co-authored-by: Mattermost Build <build@mattermost.com>

server/channels/api4/reaction_test.go

@@ -55,7 +55,7 @@ func TestSaveReaction(t *testing.T) {
 	})
 
 	t.Run("save-second-reaction", func(t *testing.T) {
-		reaction.EmojiName = "sad"
+		reaction.EmojiName = "cry"
 
 		rr, _, err := client.SaveReaction(context.Background(), reaction)
 		require.NoError(t, err)
@@ -290,7 +290,7 @@ func TestDeleteReaction(t *testing.T) {
 	r2 := &model.Reaction{
 		UserId:    userId,
 		PostId:    postId,
-		EmojiName: "smile-",
+		EmojiName: "cry",
 	}
 
 	r3 := &model.Reaction{
@@ -302,7 +302,7 @@ func TestDeleteReaction(t *testing.T) {
 	r4 := &model.Reaction{
 		UserId:    user2Id,
 		PostId:    postId,
-		EmojiName: "smile_",
+		EmojiName: "grin",
 	}
 
 	// Check the appropriate permissions are enforced.