Skip and warn on emoji import when name conflicts with system emoji (#19516)

This changes import behavior related to emoji imports when the name
conflicts with the name of a system emoji. Previously, the import
would fail, but now a warning is logged and the conflicting emoji
is skipped.

model/emoji.go

@@ -78,9 +78,12 @@ func (emoji *Emoji) IsValid() *AppError {
 }
 
 func IsValidEmojiName(name string) *AppError {
-	if name == "" || len(name) > EmojiNameMaxLength || !IsValidAlphaNumHyphenUnderscorePlus(name) || inSystemEmoji(name) {
+	if name == "" || len(name) > EmojiNameMaxLength || !IsValidAlphaNumHyphenUnderscorePlus(name) {
 		return NewAppError("Emoji.IsValid", "model.emoji.name.app_error", nil, "", http.StatusBadRequest)
 	}
+	if inSystemEmoji(name) {
+		return NewAppError("Emoji.IsValid", "model.emoji.system_emoji_name.app_error", nil, "", http.StatusBadRequest)
+	}
 
 	return nil
 }