Add some limiting to the returned results.

Angular is notoriously slow when you have a few thousand two-way
bindings active, and there's no way I can remove them. Limiting the
results returned improves things immensely, so the browser no longer
locks up.
asyncio.as_completed now complains if you give it a generator instead of a list.

Also, the fact that I was doing as_completed(as_completed(stuff)) is
enough to merit changing it.
Mixing tabs and spaces is horrible, even in HTML.