fix #1689: REST GET on /me to get user email address each time beginSession is called
check if email has been set
Merge branch 'develop' into issue/1689-use-me-endpoint-email-mixpanel

Conflicts:
	WordPress/src/main/java/org/wordpress/android/analytics/AnalyticsTrackerMixpanel.java
rename UserPrefs to AppPrefs and use an Enum instead of free strings
Merge branch 'develop' into issue/1689-use-me-endpoint-email-mixpanel

Conflicts:
	WordPress/src/main/java/org/wordpress/android/ui/reader/ReaderSubsActivity.java
Merge pull request #1783 from wordpress-mobile/issue/1689-use-me-endpoint-email-mixpanel

fix #1689 use me endpoint email mixpanel