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`.
> it's not a full sentence

I fixed that.

This same problem will happen with https://github.com/swcarpentry/workshop-template/pull/96 and I think that we can't solve it because there is no way to create a list inside a paragraph since

~~~
$ cat list.md 
Foo
1.  A
Bar
$ pandoc -t html list.md
<p>Foo 1. A Bar</p>
~~~

and

~~~
$ cat list.md           
Foo

1.  A

Bar
$ pandoc -t html list.md
<p>Foo</p>
<ol style="list-style-type: decimal">
<li>A</li>
</ol>
<p>Bar</p>
~~~