Social share buttons: Enable on story pages; on mobile stick to bottom of viewport

[benlk]

• Create issue for tracking post-deploy tasks: Post-deploy configuration tasks #9
• Re-enable social share buttons on Staging
• Using CSS, pin the social share buttons to the bottom of the viewport when the viewport is too narrow to display the buttons on the side of the article.

[benlk]

This is blocked by WPBuddy/largo#1576 making the floating social buttons unclickable, and WPBuddy/largo#1623 throwing off all the floating social button positioning calculations.