Change license to CC BY 4.0

Based on @wking's work.

## Steps to reproduce

~~~
$ cd lesson-template
$ make preview
$ cd site
~~~

Open `../lesson-template/LICENSE.html` to copy the "Instructional Material" section and replace it on `license.html`.
This line appears out of place - it's not a full sentence, doesn't follow from what's immediately above it, and doesn't lead into what follows.  Please check the original.