Fix webassets

The current setup of assets behaves as if `ASSET_DEBUG=True` because it fails at finding the assets and the static file builtin server saves the day.

* Updating `Flask-Assets` to 0.9
* Fixing the usage of `webassets` so it combines the bundles
* Adding a configuration key for serving static via a dedicated host / CDN: `STATIC_URL_PREFIX` (set to `/` by default).
* Enhancing the documentation with sphinx syntax (mostly `code-block`)
* Adding `ext/assets` to the documentation.