Kill Flask.

...and use Werkzeug directly.

The logic is pretty much the same. Most of the decorators have moved to
a new module, haas.http. haas.api has otherwise barely changed.

Along the way I cleaned up various error messages and added some
comments that aren't strictly related to this patch -- they just
happened to be needed in code that I ended up touching.

This could certainly use more testing, and if nothing else I want to run
the deployment tests before merging.