Adds profile, list, search, and collection timelines. Upgrade Twitter advertising pixel to universal website tag.

• Embed a profile timeline, list timeline, or collection by pasting a URL, customizing a shortcode, or a widget
• Embed a search timeline by shortcode or widget using a widget ID configured on Twitter.com
• Upgrade Twitter advertising tracker to universal website tag
• Twitter Cards include image alternative text when available
• Follow button and Periscope On Air button use post author or site username if no username specified
• Improved compatibility with WordPress.com / Jetpack formatting of tweet shortcode
• Prefer wp_resource_hints API for DNS prefetch in WordPress 4.6+
• Update single Tweet with video to remove status customization no longer supported by Twitter
• Fix bug when a large featured image is highlighted in a Twitter Card
• Describe site username option for WordPress REST API in WordPress 4.7
• Bump minimum WordPress version to 4.1