Ensuring license URL is valid, adding CC0 url. Fixes #440

Sets the license url to null if the license isn't listed on opensource.org or in the alternative licenses list I added (currently just CC0).

Updated the template to just display the name of the license without a link if no URL is available.

On my laptop, 11 tests failed both before and after this commit, so no new failures. The VM is still downloading but I'm 80% sure that it will work ;)
Could use something like [osi-licenses](https://github.com/meryn/osi-licenses) for this?