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 were 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 that nobody has complained about it), even though it's not necessarily ideal.

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