Add remove-short-tags grunt task

Let's introduce the remove-short-tags grunt task, that replaces short php tags (eg `<?` or `<?=`) with long tags (eg `<?php ` or `<?php echo `).

With the provided `short-tags-remover.php` we no longer need these workarounds:
- https://github.com/concrete5/concrete5/blob/master/web/concrete/blocks/date_nav/tools/rss.php#L22
https://github.com/concrete5/concrete5/blob/master/web/concrete/blocks/page_list/tools/blog_rss.php#L28
https://github.com/concrete5/concrete5/blob/master/web/concrete/blocks/page_list/tools/rss.php#L29