Add improved new-account flow.

This is an improved new-account flow.

It goes like this:

(1) "Let's play" now goes to gym.pencilcode.net
(2) There is a new "New Account" button for cookieless users (it shrinks to a small "New account" link when a user has a cookie already).
(3) And to go along with that button, there is a new version of the login dialog that has text that says "Create new account."
(4) And to bring this dialog up, there is support for a new URL hash #new, so if you visit pencilcode.net/edit/intro#new, it will automatically bring up the Create-new-account dialog.  The welcome page's "new account" link links to this URL.


I think there might still be value, for some users, of the old gallery of working, finished programs, just for the "what can I do with this" possibility; not all of these have obvious unique exercises associated with them.  Maybe some kind of "I'm feeling lucky" button on gym.pencilcode.net?