Minor UX issues in Safari.

[kwight]

Originally started by @beaulebens in https://github.com/Automattic/notifications-client/issues/229:

• In Landscape mode, the Notifications panel maxes out at some width which is less than the full width of the screen. Maybe when in single-column mode (when the Masterbar changes layout), we should make the Notifications panel 100% width, so that in that "mode", it always covers the full screen?
• Going from landscape to portrait mode results in the text being much larger (and unreadable). Closing and reopening the notifs panel gets it back to normal.