fix(river): add enabled col to river table, update on enable/disable of ...

...referenced entities

Fixes #6022 - empty river items fetched when referenced entities are disabled

TODO:
- [x] Make events private API
- [x] Fix Comments for Travis
- [x] Manual Testing
- [x] Automated Tests
- [x] Add the column to the install schema
- [x] Add documentation for property to model class
- [x] Export property value in toObject
Since these functions are handling "all" disable/enable events, I think they should first check that $entity instanceof ElggEntity before proceeding. (We shouldn't assume only entites will have disable/enable events, right?)