Make phrasing on "Mark All Read" consistent

Summary: Fixes T5025. We label this action in two different ways ("Clear All Notifications", "Mark All Read"). The latter is probably a better description.

Test Plan: looked at the UI.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T5025

Differential Revision: https://secure.phabricator.com/D9073