Fix config license change, add support for custom license_url

[froggleston]

This PR fixes #619 by:

• properly using the license key in a lesson config.yaml
• adding a new config option license_url so that users can supply custom license URLs

There are various config.yaml options:

• If the license is omitted completely, it will default to CC-BY 4.0, as before, and the footer license href will point to LICENSE.html
• If a license is supplied, but no license_url is supplied, the footer license href will point to LICENSE.html
• If a license is supplied and a license_url is supplied, both are used together

[ErinBecker]

I've tested both changing the 'license' variable and setting a license_url' variable and both work as described.