Fixed #20500 - Updated flatpages URLconf example to work with APPEND_SLASH.

Thanks josh.23.french@.
Fixed #20462 - Fixed sqlite regex lookups for null values and non-string fields.
Added docs subdir to gitignore

To be able to build translated documentation, docs/locale should
be a symbolic link to the django-docs-translations/translations
directory.
Revert "Fixed #20462 - Fixed sqlite regex lookups for null values and non-string fields."

This reverts commit 64041f0e6e7a773574d7cde978d16305200004ea.

lookup.tests.LookupTests.test_regex_non_string fails under Postgres.
We should also try to rewrite the test using an existing model.
Fixed #20591: added version in docs for utils.module_loading.import_by_path.
Merge pull request #1262 from batisteo/ticket-20591

Fixed #20591: added version in docs for utils.module_loading.import_by_path
Merge pull request #1259 from Wilfred/master

Simplified a formset example in the docs
Fixed #20579 -- Improved TransactionTestCase.available_apps.

Also moved its documentation to the 'advanced' section. It doesn't
belong to the 'overview'. Same for TransactionTestCase.reset_sequences.

When available_apps is set, after a TransactionTestCase, the database
is now totally empty. post_syncdb is fired at the beginning of the next
TransactionTestCase.

Refs #20483.
Warned that `request_finished` isn't sent by some buggy setups.

Older versions of uWSGI and Sentry's middleware do not adhere to
the WSGI spec and cause the `request_finished` signal to never
fire. Added notes to the appropriate places in the docs.

Fixed #20537.