Adds potential fix and logging for issue #1945

I tried to reproduce #1945 with no success. I did have a look at the code though, and noticed that we were loading loading `PostsActivity` from the notification, which seemed unnecessary so I change it to load `NotificationsActivity` directly instead.

Also added some additional logging and a fix for #1948.
Since we're no longer checking for FROM_NOTIFICATION_EXTRA here, can that constant be removed from NotificationsActivity?