Allow serving static content from a CDN

This is controlled by STATIC_HOST. For example, set

    STATIC_HOST=mydomain.cloudfront.net.
Clear config environment variables after reading them

We use environment variables for configuration. But they don't need to
persist after application startup, so delete them.
Inline main.js code in template

main.js is awkward because it relies on the prices() function to be
previously defined. So it didn't fit into the asset pipeline.

And now that we support CDNs, we don't want to deal with cache
invalidation for main.js. So just inline it in the main.html template.
Make collector run forever

This is more consistent how Procfile processes are supposed to
behave. The old behavior is still available with the --once flag,
which makes it suitable for running from Heroku Scheduler.