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
Creating a new DateFormat every time this is called can be expensive. A better approach would be to create a static thread-safe formatter than can be reused, like this:
```
private static final ThreadLocal<DateFormat> AnalyticsDateFormat = new ThreadLocal<DateFormat>() {
    @Override
    protected DateFormat initialValue() {
        DateFormat format = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss'Z'", Locale.US);
        format.setTimeZone(TimeZone.getTimeZone("UTC"));
        return format;
    }
};

public void setCurrentDateForPeopleProperty(String property) {
    setPeoplePropertyToValue(property, AnalyticsDateFormat.get().format(new Date()));
}
```