Restrict Workboard initialization to CAN_EDIT

Summary: Make Workboard initialization more restrictive.

Test Plan: Log out, see "No Workboard", Log in with permissions, see "New Workboard", Log in with notchad, see "No Workboard".

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T7410

Differential Revision: https://secure.phabricator.com/D14306