Added in some missing analytics

The iOS project was getting a bit farther ahead of Android in terms of Analytics, so I made this PR to bring them closer to parity. Some of the changes include:
* Adding in "Editor Closed" and "Editor - Saved Draft" events (https://github.com/wordpress-mobile/WordPress-iOS/pull/2044)
* Flagging Users who Create an Account on Mobile (https://github.com/wordpress-mobile/WordPress-iOS/pull/1991)
* Added a "Sign In" event (https://github.com/wordpress-mobile/WordPress-iOS/pull/2062)
* Correctly flagging a user who signs in as a dotcom_user
* Fixed bug where `number_of_blogs` super property wasn't being set properly(https://github.com/wordpress-mobile/WordPress-iOS/pull/2043)
* Added in a series of people properties to better track when users last performed specific actions(https://github.com/wordpress-mobile/WordPress-iOS/pull/1586 and https://github.com/wordpress-mobile/WordPress-iOS/pull/2082)
* Added tracking for Theme Previews
Yea, it's a special Mixpanel property.