Social Share buttons + 1st attempt to fix render blink.

It's an external link, though? And GitHub just redirects http to https immediately anyway.

I've usually seen `//` for external resources in `<link>` or `<script>` tags... although there isn't much reason to not just use `https` for all of them; you don't get any errors linking to `https` resources from an `http`-served page, after all.