[bug 1071959] Add share CTA macro for 10th anniversary pages

Import macro on any given template:

    {% from "macros.html" import share_cta with context %}

Example usage:

    {{ share_cta('Share the video', 'https://www.mozilla.org', 'Share text' }}

Additional files:

* mozilla-share-cta.less
* mozilla-share-cta-ie.less (fallback styles for IE8 if needed)
* mozilla-share-cta.js

Does this result in an extra copy of the `lib.less` styles being included/downloaded? I know it needs that reference to understand our mixins and variables...could we maybe force implementers to include this `.less` file in the including page's `.less` file (which presumably includes its own copy of `lib.less`)?