Add "Play online now" buttons to all the index pages.

I feel like the old pages weren't as obvious as they could
have been about the fact that yes, you can start playing
immediately online. Let's see if this helps.

I've also condensed the adv551 intro text, and updated it
to match my current best guess about MCDO0551's history.
Add HTML content for "Adventure II".

And fix a bunch of bugs in the cut-and-pasted HTML
for all the other versions.