Added a (somewhat clumsy) polling mechanism to the create link page. Now you
get a spinner until your screen cap is created. We poll to check for completion
every two seconds. Our create JS is getting pretty crufty. We should clean it
and the related templates in create-link.

fixes #446
bug fix: Now that we're generating the screen cap async, we need add a "loading"
state (spinner with non-clickable tab) to the single link view. fixed.

The spinner and pending logic on the single link page are pretty crufty. Way
complex. We should do a bit of refactoring in that view.