[1.5.x] Allow build of translated documentation

Backport of 6a092f24e from master.
[1.5.x] Fixed #20567 - Documented BoundField.id_for_label.

Thanks littlepig for the suggestion.

Backport of 175a102ddc from master.
[1.5.x] Fixed #19875 - Added warnings regarding DEBUG=False and empty ALLOWED_HOSTS

Backport of 4e94c84e50 from master
[1.5.x] Fixed #20578 - Typo in BaseFormSet module name

Backport of 9ff5f100dd from master
[1.5.x] Fixed #20500 - Updated flatpages URLconf example to work with APPEND_SLASH.

Thanks josh.23.french@.

Backport of 536703abf0 from master
[1.5.x] 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.

Backport of 3ce1d30.

Conflicts:
	docs/howto/deployment/wsgi/index.txt
[1.5.x] Fixed an erroneous import in example code.

Backport of bd68f701b from master. Fixes #20595.
[1.5.x] Fixed #20604 - Fixed file example in docs.

Thanks Chris Lasher for the report.

Backport of a1122e14a6 from master
[1.5.x] Fixed #20614 -- Typo in documentation.

Backport of 39b245fd76 from master
[1.5.x] Fixed #20615 -- Added trove classifiers for major Python versions.

Backport of efe6e160.
Fixed broken reference in documentation.