Add support for Sandstorm web key URLs

This commit adds support for Sandstorm style web key URLs.

You've probably never heard of Sandstorm (https://sandstorm.io/), but it's a project I work with whose purpose is to make webapps such as Tiny Tiny RSS easy to setup and host by end users. All apps are run inside a sandbox, and so handling incoming connections is a bit tricky. The way it works for API access such as this is a key is provisioned per app instance, and provided to the user as a web key of the form http://$HOST#$KEY. Client apps such as this android client will then take this url, parse out the key, and pass along the key in the Authorization headers a Bearer token (similar to OAuth).

If you're interested in checking out Tiny Tiny RSS inside Sandstorm to see how it works, you can head over to https://demo.sandstorm.io and find it in the "Install Apps" list.