Remove hovercards from Feed panels on dashboards

Summary:
Fixes T5472.

I do imagine doing a pass on the Hovercard JS at some point to try to make them position more intelligently (I've hit a few cases where they do something silly, and we can probably fix many of them), but generally agree that this is inconsistent and questionably valuable on panels.

Test Plan:
  - Moused over feed stuff in a panel, no hovercards.
  - Moused over feed stuff in Feed, got hovercards.

Reviewers: chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T5472

Differential Revision: https://secure.phabricator.com/D9753