Use shorthand string appending
Use VIP-safe remote data fetching methods
Remove code that is never executed
Return void instead of false

Maintains consistency with the other return statement in the function.
Add proper escaping to admin settings fields
Remove activation hooks, move contents to constructor
Code style (whitespace) changes
More code style changes

From PHPCS feedback (WordPress VIP standards)
Escape all the things
Code style (whitespace) changes
Add PHPDoc comments
Code style changes

Double quotes to single where appropriate, comma after final item in an
array
Exit PHP instead of echoing an HTML string

Maintains consistency with functions similar to this one. Also looks
nicer.
Consolidate duplicate HTML class attributes
Remove sync.php

Direct access scripts are a no-no on WordPress.com VIP.
Move HTML templates from options to functions

With HTML templates stored in the plugin's option value, the size of
that option could grow as large as a user wanted it to. Taking the HTML
out of the options limits the size of the option value.

Filters were added so the HTML could still be customized from within a
theme.
Replace db GET_LOCK with a transient
Remove $wpdb declaration (no longer being used)
Clean up error handling code and add todos

Also it needed some adjusting for the post meta to get saved properly.
Merge branch 'vip-feedback-changes'