Post vote

# Changes
 - Adds Facebook and Twitter share links.
 - Breaks "share" component out of footer so we can use same link style elsewhere.
 - Adds code to prevent double form submission.
 - Adds disabled & :rotating_light: _danger_ :rotating_light: button styles.

# Notes
Facebook doesn't seem to let us set default share text through either of their Share APIs. Is this a problem?