Retry failed page anchor navigation

Summary:
When the user loads a page with an anchor on it like #thing, or clicks a link to
#thing, and #thing doesn't exist, keep trying to navigate to #thing for a few
seconds.

This allows anchors to work when the target is in content which is later ajaxed
in. In particular, this affects inline comments in Differential.

Test Plan: Opened inline comment links in a new tab, was in the right place when
I switched tabs.

Reviewers: nh, btrahan, jungejason

Reviewed By: btrahan

CC: aran, btrahan, epriestley

Maniphest Tasks: T492

Differential Revision: https://secure.phabricator.com/D1327