When viewing a workboard, only link to parent workboards in crumbs if parents have workboards

Summary:
Depends on D20516. See PHI1247. In D20331, I made the crumbs on workboards point at ancestor workboards.

However, this isn't a great destination if an ancestor doesn't actually have a workboard. In this case, point at the normal profile URI instead.

Test Plan:

• Viewed a milestone workboard with a parent that had no workboard. Saw a profile link instead of a workboard link (new behavior).
• Viewed a milestone workboard with a parent that also had a workboard. Saw a workboard link (existing old behavior still works).

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20517