[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