[MM-9777] Avoids adding custom emojis with the same name as a system emoji (#8461)

* Emoji validation fails if name exists in system emojis

* Use hashmap instead of array to improve performance

* Changed utils/StringInMap to emoji/isSystemEmoji

* Load system emojis from model/emoji.json

* Added emoji.json from webapp

* Load system emojis from emoji_data.go instead of emoji.json

* Run `gofmt -w model/emoji_data.go`

model/emoji_test.go

@@ -80,4 +80,9 @@ func TestEmojiIsValid(t *testing.T) {
 	if err := emoji.IsValid(); err == nil {
 		t.Fatal(err)
 	}
+
+	emoji.Name = "croissant"
+	if err := emoji.IsValid(); err == nil {
+		t.Fatal(err)
+	}
 }