Projects - re-label "Workboards" as "Workboard"

Summary: better aligns user expectations with product reality. Fixes T6391.

Test Plan: viewed a project and saw "workboard". viewed some project search results in /project and saw "workboard".

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T6391

Differential Revision: https://secure.phabricator.com/D10747