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