[bug 766394] Generate share links for KB articles.

Share links are generated via bit.lys API, and the configuration
for accessing that API can modified in settings.py.
[bug 766394] Added show/hide share link option.

Added migrations, form element, and now share_links are auto-generated
for each new KB article, but you can choose to hide or show them.
[bug 766394] Fixed styling for share link button.