Use sprites for all the menu icons

Summary:
See D3277, D3278.

• Sprite all the menu icons.
• Delete the unsprited versions.
• Notification bolt now uses the same style as everything else.

Test Plan: Looked at page, hovered, clicked things.

Reviewers: btrahan, chad, vrana

Reviewed By: vrana

CC: aran

Maniphest Tasks: T1569

Differential Revision: https://secure.phabricator.com/D3279