Fixed typo in docs/howto/custom-lookups.txt.
Added an "htmlview" target to open docs after building them.

This is helpful if you build and test documentation updates in
your development env.

Copied from CPython's Doc/Makefile.
Fixed #23968 -- Replaced list comprehension with generators and dict comprehension
Refs #23964 -- Added warning about case-insensitive, unique fields used with formsets
Fixed #23974 -- Clarified wording of FileField.save/delete save parameter.

Thanks GreenAsJade.
Fixed #20968 -- Checked Spatialite metadata before migrations

Thanks Kenial S. Lee for the initial patch and Tim Graham for
the review.
Ran 'CREATE EXTENSION postgis' during prepare_database hook

DatabaseWrapper.prepare_database has been introduced in 307de67073.
Fixed typo in spatialite.txt

Thanks Tim Graham for spotting the error.
Remove text-mangling of default values.
Fixed #23961 -- Clarified when makemigrations will create a directory.
Refs #23947 -- Prevented staticfiles tests from having side effects.
Fixed incorrect filter name in docs/topics/i18n/translation.txt.
Fixed #23956 -- Fixed migration creation for multiple table inheritance
Fixed typo in admin deprecation message.
Fixed #23977 -- Added setdefault() method to HttpResponse
Appeased spell checker.
Cleanup cache in contrib.sites to prevent test interference -- refs #11505
Refs #18586 -- Split custom_pk test.
Refs #18586 -- Split custom manager tests.
Readded test inadvertently removed in last commit.