We already have `$post` as a global above

No need to call `get_post()` here.
We should use the feedback's parent, not ~
Only display the URL if we have one.

If we don’t have one, it could have come from a widget.
Make sure we're escaping the URLs correctly.
Merge pull request #705 from Automattic/grunion-form-url

Resolves feedback list link 404. Closes #658