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.

app/import_functions.go

@@ -1813,8 +1813,13 @@ func (a *App) importMultipleDirectPostLines(c *request.Context, lines []LineImpo
 }
 
 func (a *App) importEmoji(data *EmojiImportData, dryRun bool) *model.AppError {
-	if err := validateEmojiImportData(data); err != nil {
-		return err
+	aerr := validateEmojiImportData(data)
+	if aerr != nil {
+		if aerr.Id == "model.emoji.system_emoji_name.app_error" {
+			mlog.Warn("Skipping emoji import due to name conflict with system emoji", mlog.String("emoji_name", *data.Name))
+			return nil
+		}
+		return aerr
 	}
 
 	// If this is a Dry Run, do not continue any further.