Make a proper API for notes

[Zverik]

Notes API is awful. Why ID in a separate tag? Why URLs and HTML? Why tags in comments, and not attributes? I understand it was to quicken front-end development, but it is no public API.

I propose to keep old one under /notes and make a new, proper API under /note (singular, re: /node, /changeset).

It should be in line with a format for planet notes dump.

[tomhughes]

Much of the stuff you object to is there to support the old web site frontend, and isn't needed anymore.

I'd rather fix it as part of the next API version though, rather than change the URLs.