Go directly to `NotificationsActivity` when tapping a push notifications.

Added private member for maximum inbox items in a notification.
Added some additional logs to help pinpoint #1945 issue.
Restores the reply icon, accidentally removed in https://github.com/wordpress-mobile/WordPress-Android/pull/1924

Fixes #1948
* Nuked unused `FROM_NOTIFICATION_EXTRA`
* Added `mShouldFocusReplyField` to track whether to focus the reply field when loading `CommentDetailFragment` from the reply action in a push notification.
Merge pull request #1949 from wordpress-mobile/issue/1945-notifications-blank-app

Adds potential fix and logging for issue #1945