Simplify ModerationManager.post_save_handler.

Restructure the code to reduce nesting and improve readability.
Simplify ModeratedObject._moderate method.

The original method was very unclear about what it was doing, and also
saved the base object more often than was necessary. I added a lot of comments
to explain what's happening, and restructured the code so that the variables
and the reasons for following certain branches are clearer and the code is
more readable in my view.

I also avoid calling self.changed_object.save(), and call save_base(raw=True)
instead to avoid triggering the pre_save and post_save hooks again.
Refactor model tests to reduce duplicated code.

Most of the tests for custom user models were just copied and pasted from
standard user models, with minor changes. Now all tests are shared between
both. There shouldn't be any important difference in what's actually tested.
Don't save ModeratedObject twice in automoderate().

approve() and reject() save the ModeratedObject anyway, so we don't need to
save it a second time.
Merge pull request #115 from aptivate/optimise_saving_moderated_object

Don't save ModeratedObject twice in automoderate().
Merge pull request #114 from aptivate/simplify_moderatedobject_moderate

Simplify ModeratedObject._moderate method.
Merge pull request #113 from aptivate/simplify_post_save_handler

Simplify ModerationManager.post_save_handler.
Merge pull request #119 from aptivate/dry_out_models_tests

Refactor model tests to reduce duplicated code.
Hide rejected objects if never approved.

The old behaviour is that rejected objects will always be displayed in
querysets, which is weird.

In my view, if an object has never been approved, it should be hidden. If
it has been approved, but a change to it is rejected, then the change should
be reverted and the object should continue to be visible, as it was when it
was approved.

This patch implements that behaviour by using the undocumented
moderation_state column to put objects that have never been approved into
DRAFT state, which hides them from the public but allows the admin to see
them. Once an object has been approved once, its state becomes READY, and
rejections don't change it back to DRAFT, they simply undo the changes.

Note that this changes the behaviour of the moderation_state field slightly.
In particular, the admin will be invited to moderate objects in DRAFT state,
which previously they were not.