Fixed #20484 again -- added note to field documentation

The bug was already fixed by 01b9c3d5,
so only tests added.

At the same time promote_joins()'s uncoditional flag is gone, it isn't
needed for anything any more.

The `MultiValueField` class gets a new ``require_all_fields`` argument that
defaults to ``True``. If set to ``False``, individual fields can be made
optional, and a new ``incomplete`` validation error will be raised if any
required fields have empty values.

The ``incomplete`` error message can be defined on a `MultiValueField`
subclass or on each individual field. Skip duplicate errors.

Sphinx generates left single quotes for apostrophes after
code markup, when right single quotes are required. The
easiest way to fix this is just by inserting the unicode
character for a right single quote.

Instances of the problem were found by looking for
">‘" in the generated HTML.

Fixed #16986 -- Model.clean() can report errors on individual fields.

Also PEP8 + python_2_unicode_compatible cleanup done.

This commit provides the tests for this issue but the actual problem was solved
by the ValidationError refactor in f34cfec0 and ee77d4b2.

Refs #20199.

Thanks ea2100@ for the report and akaariai for the patch.

Added optional html_email_template_name parameter to password_reset view
and PasswordResetForm.

Thanks Keryn Knight.

Refs #17008.

Refs #18681.

This also starts the deprecation of ModelAdmin.declared_fieldsets

Fixed #20855 -- Added documentation of current_app and extra_context params to django.contrib.auth views

refs #5298 and refs #8342

Moved get_search_results from BaseModelAdmin to ModelAdmin.

Thanks Loic Bistuer for the review.

Refs #15961.

Thanks AtomicSpark.

Thanks Jimmy Song for the patch.

Thanks Collin Anderson for the report.

Regression introduced by [369b6fab]; refs #18169.

Refs 23e1b59c which already fixed this issue for ModelAdmin and InlineModelAdmin.

Thanks Daniele Procida for the suggestion and edits.

Thanks croldan for the report.

Thanks thibaultj for the report, jenh for the patch,
and charettes for the tests.

Forward-port of 3493f18d from master.