Add a basic first feed story on /home/

Summary: Ref T11132. Adds a text panel to feed if no stories are present and the user is an admin. Seems ok-ish for 15 minutes. Happy to take content suggestions.

Test Plan: Make a new install, see panel. Log in as new user, don't see panel.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T11132

Differential Revision: https://secure.phabricator.com/D16479