Provide iframe page embeds

[reefdog]

The time has come. We've decided to accept a fixed-aspect-ratio page embed. We need to make a few changes to make this happen:

• Move wrappable items (title/logo header and credit footer) out of page view so that they only exist in the embed markup
• Make menubar minimally usable for those who hack in pageNavigator or text mode
• Ensure notes don't pop out of page bounds
• Serve embed pages from platform

There will be further improvements, particularly to notes, but those can come post-MVP.