Shows an alert if the user opens in preview mode a post with unsaved changes.
Merge pull request #2867 from wordpress-mobile/issue/editor-426-show-alert-for-unsaved-changes

Shows an alert if the user opens in preview mode a post with unsaved changes.
Merge branch 'release/4.7' of github.com:wordpress-mobile/WordPress-iOS into issue/editor-406-confirmation-alert

# By Diego Rey Mendez
# Via Diego Rey Mendez
* 'release/4.7' of github.com:wordpress-mobile/WordPress-iOS:
  Shows an alert if the user opens in preview mode a post with unsaved changes.
SVProgressHUD will now display after post/page saves.