Show the recipe input form in a modal dialog

I tried hard to do this, but it turned out to be damn near impossible because trying to make this work puts you right in the middle of a fistfight between the Form API, Batch API, and AJAX API. A fight which nobody's winning, but you are definitely losing.

I discussed this with @tim.plunkett, maintainer of the Form API, and he felt that this is too much of a Pandora's box to justify the slightly improved usability. What we have now works just fine, in the sense that nobody has complained about it, even though it's not necessarily the ideal UX.

So I'll postpone this indefinitely and leave it for a braver (and/or foolhardier) and more skilled keyboard warrior than myself.

Well sheeeeit. @pameeela did her sorcery thing when I told her this couldn't be done, and wouldn'tchaknow, I found a way: OpenModalDialogWithUrl, a core AJAX command that does what we want (by inadvertently making the form action not point to an existing AJAX route, which is why this was breaking before).

This is now fully on.

Here's a screenshot of what the form now looks like.