New Sharedaddy: Harden icon-only look against CSS bleed, fixing oval share buttons on some sites.

Merges r93955-wpcom.
New Sharedaddy: Add on fix for CSS hardening on icon only sharing buttons.

Merges r93956-wpcom.
Markdown:

* improve `<code>` matching regex
* preserve `<code>` elements before shortcodes to handle the edge-case where a shortcode like `[[sourcecode]]` is inside a `code` block

Fixes #6349

Merges r93987-wpcom.
Shortcodes CSS: Use correct indentation