Randomized KEY_PREFIX in caches test to prevent failures during parallel testruns.
Fixed #21505 -- Added unit test for django.utils.text.get_valid_filename.
Fixed regression from ffc37e2343a93cf6d44247e20cd263b41f931716.

This (hopefully) ensures that the cache are created the same way as before
the offending commit.
Don't fail if there is no memcached backend active.
Fixed #21504 -- Don't shadow database errors when testing for postgis version.

Thanks to trac user paultag for the report.
Fixed flake8 error (5 space identation!!!)
Properly closed cache connections at the end of the request.

This only affects the new cache api and not the deprecated get_cache.

Refs #21012
Fixed #21500 -- Removed imports of deprecated utils.importlib
Added more tests for ContentTypeManager.get_for_model.
Fixed #21497 -- Forced conversion to bytes for very long index names
Removed confusing comments from the docs.

The settings reference documentation doesn't seem the right place to
invite users to write their own db and cache backends.

Also the actual wording makes the task sound trivial, which is hardly
the case; writing a custom db backend is a very difficult task, and
writing a custom cache backend is full of gotcha.
Fixed #21490 -- Fixed custom admin URL reverse example.

Thanks glarrain for the report.
Fixed typo in previous commit; refs #21490.
Fixed #21391 -- Allow model signals to lazily reference their senders.