Add available_objects soft deletable model manager

[craiga]

Problem

As described in #364

Solution

• Adds .available_objects manager to soft deletable models.
• Keeps existing .objects manager, but emits a DeprecationWarning for each use of that manager.

Commandments

• Write PEP8 compliant code.
• Cover it with tests.
• Update CHANGES.rst file to describe the changes, and quote according issue with GH-<issue_number>.
• Pay attention to backward compatibility, or if it breaks it, explain why.
• Update documentation (if relevant).

[craiga]

Still needs documentation. Just hoping for feedback on this approach for now.

[jonathan-s]

This approach looks sane to me. Perhaps the link should be to the documentation instead for this? With regards to emitting too many warnings, I believe there is a way to silence warnings which could be mentioned in the documentation. If for nothing else this will bring people the notice of how things will work in the future and change things accordingly.

[craiga]

Test named incorrectly.

[rjekker]

One thing is not clear for me:

does this mean that in the future, objects will again be the default Manager that returns everything?

What is currently the recommended way to get to deleted items when using SoftDeletableModel?

[craiga]

@rjekker Yep, that's right—objects will be a plain old simple manager.

I might be wrong, but there is no recommended way to get deleted items at the moment. That's part of the underlying issue this deprecation is trying to solve.

[tadamcz]

Thanks for moving this along, @craiga! I added some clarification to the docs (#589) as I was initially confused.