Adding hovercard support for object tags

Summary: Refs T1048 - Adds sigil and metadata to emitted tags.

Test Plan: Inspected HTML, verified sigil and meta data reference. Hooked up behaviours in a future broken diff. Could successfully load hovercards from endpoint. I'm bad with JS though.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T1048

Differential Revision: https://secure.phabricator.com/D5558