Into a component that does Interactive Auth and a dialog that
wraps it, so we can do interactive auth not necessarily in a
dialog.
As a side effect:
* Put the buttons for each auth stage in the stage itself.
Some stages don't have submit buttons (and it's very possible
other stages may have other buttons entirely, like 'resend')
so it makes more sense for the buttons to live in the stage
components themselves. Plus it saves the slightly evil
calling-functions-on-react-children thing we were doing (and
indeed extending that to show the submit button at all).
* Give all BaseDialogs a cross in the top right to cancel. They
were all dismissable by clicking outside or pressing esc, so
this adds a more visually obvious way of dismissing them. Plus,
it means our InteractiveAuthDialog can have a way of canceling
the whole operation separate from buttons for the individual
stages.
