We used to have app/models/activity in the autoload path and had classes
like FriendActivity < Activity

This conflicts with how rails4 handles class lookup. Plus it clutters the
autoload path and makes it harder to find classes.

Using proper namespaces instead now and I'm about to apply this to all the
different namespace avoiding things like ...Extension. We can use classes
as namespaces as long as we:
* make sure to define things inside the namespace in one line or in a
   class instead of a module block:
   ```ruby
     class Activity::Friend < Activity
     # or
     class Activity
       class Friend < ::Activity
   ```
 * have names inside the namespaces that do not conflict with global names

The second one is important because if PrivatePost is already loaded and
Activity::PrivatePost is referenced somewhere it will be found as an
constant inside the Object namespace of Activity (since Activity is a class).
So instead of loading Activity::PrivatePost we'll see a warning that global
constant PrivatePost was referenced by Activity::PrivatePost.

So how do we prevent this?
* do not clutter the global namespace
* use more specific names in other namespaces
  -> in this case i am renaming PrivatePostActivity to Activity::MessageSent
  and not Activity::PrivatePost