IE8 dialog resize fix

Not quite ready, needs feedback. @shane-tomlinson mind having a look?

The problem with resizing the dialog on IE8 is that IE8 never tells you the actual height of the dialog, including browser chrome. But, if you tell IE8 to resize the window, it thinks you mean window+chrome, so you wind up with squished contents. Oddly, setting a height in window.open does not include the browser chrome, hence our current headaches.

Rather than ask IE8 to resize the window to a height of 375, instead we (1) record the actual height, then (2) reset it to that height later.

I'm out of time for today and will spin up a bigtent instance tomorrow to test this in an IE8 VM, but wanted to get feedback on the general approach sooner than later.

It turns out there are two places where resizing happens. One of them is the error page, mentioned by @jrgm in mozilla/browserid#3476. The other is when signin completes, which is the similar error I found when trying to repro earlier today. Both cases are handled here, and I tried to be clean about namespacing the code without going nuts and cleaning up everything.

Feedback welcome.