MM-41728: Validate emoji size in app while importing (#20325)

* MM-41728: Validate emoji size in app on import

* use NotNil instead of Error for AppErr

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

app/import_functions.go

@@ -1867,7 +1867,8 @@ func (a *App) importEmoji(data *EmojiImportData, dryRun bool) *model.AppError {
 	}
 	defer file.Close()
 
-	if _, err := a.WriteFile(file, getEmojiImagePath(emoji.Id)); err != nil {
+	reader := utils.NewLimitedReaderWithError(file, MaxEmojiFileSize)
+	if _, err := a.WriteFile(reader, getEmojiImagePath(emoji.Id)); err != nil {
 		return err
 	}