Restore a more surgical workflow/target check

Summary:
D3707 removed some overbroad rules for when we trigger workflows, but there is still one case we should check for -- an <a /> without workflow inside a <form /> with workflow. This occurs in, e.g., the "help" button in Remarkup.

If the node with workflow isn't a link, don't trigger workflow. This should allow the <a><img /></a> case to keep working properly.

Test Plan: Clicked "?" in remarkup bar.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D3722