Fixed #22006 -- Documented how to write a login_required mixin for CBVs.

Thanks django at patjack.co.uk for the suggestion and mockforest
for the draft patch.
Fixed #21179 -- Added a StreamingHttpResponse example for CSV files.

Thanks charettes for the suggestion.
Amended login required mixin documentation to honor **initkwargs.
Fix any sqlite field migration deleting all implicit m2m tables

Fixes #22281
Fixed #22283 -- Updated documentation referring to non-existent tuples.
Mentioned that ``dumpdata --natural`` is deprecated.
Made a test app label unique.
Fixed a regression from 2e5b82f6277227b69e9a6c5261a6deb14fff15dc
Fixed regression in file locking on some platforms.

Some platforms with os.name == 'posix' do not have the
fcntl module, e.g. AppEngine.

refs #19373.