Deprecate Django returner

[waynew]

What does this PR do?

Deprecates the Django module so we can remove it. TBH we could probably just go ahead and remove it.

It's likely that the Django returner never worked - it can only fire signals from within the same process that the Django server is running in, which won't be the process that Salt is running in. Deprecating and marking for removal.

Merge requirements satisfied?

[NOTICE] Bug fixes or features added to Salt require tests.

• [x] Docs
• [x] Changelog - https://docs.saltproject.io/en/master/topics/development/changelog.html