Nested deconstruction should (silently) handle Field.deconstruct() as
well as other arbitrary deconstructable objects. This allows having a
field in the deconstruction of another field.

The first part of django.contrib.postgres, including model and two form
fields for arrays of other data types.

This commit is formed of the following work:

    Add shell of postgres app and test handling.

    First draft of array fields.

    Use recursive deconstruction.

    Stop creating classes at lookup time.

    Add validation and size parameter.

    Add contained_by lookup.

    Add SimpleArrayField for forms.

    Add SplitArrayField (mainly for admin).

    Fix prepare_value for SimpleArrayField.

    Stop using MultiValueField and MultiWidget.

    They don't play nice with flexible sizes.

    Add basics of admin integration.

    Missing:
    - Tests
    - Fully working js

    Add reference document for django.contrib.postgres.fields.ArrayField.

    Various performance and style tweaks.

    Fix internal docs link, formalise code snippets.

    Remove the admin code for now.

    It needs a better way of handing JS widgets in the admin as a whole
    before it is easy to write. In particular there are serious issues
    involving DateTimePicker when used in an array.

    Add a test for nested array fields with different delimiters.

    This will be a documented pattern so having a test for it is useful.

    Add docs for SimpleArrayField.

    Add docs for SplitArrayField.

    Remove admin related code for now.

    definition -> description

    Fix typo.

    Py3 errors.

    Avoid using regexes where they're not needed.

    Allow passing tuples by the programmer.

    Add some more tests for multidimensional arrays.

    Also fix slicing as much as it can be fixed.

    Simplify SplitArrayWidget's data loading.

    If we aren't including the variable size one, we don't need to search
    like this.

Moved ServerHandler helper class to tests.

Fixed #22421 -- Regression in fixtures loading.

Added ModelForm and NON_FIELD_ERRORS imports.

Previously the FORMAT_MODULE_PATH setting only accepted one string (dotted
module path). A feature has been added to also allow a list of strings.

This is useful when using several reusable third party apps that define new
formats. We can now use them all and we can even override some of the formats
by providing a project-wide format module.

Thanks Anssi Kääriäinen and Tim Graham for the reviews. Refs #17001 and #22650.

Regression from f51c1f59 when using select_related then prefetch_related
on the reverse side of an O2O:

Author.objects.select_related('bio').prefetch_related('bio__books')

Thanks Aymeric Augustin for the report and tests. Refs #17001.

#22667 replaced occurrences of master/slave terminology with leader/follower

When using ManifestStaticFilesStorage, deleted static files would be
correctly cleaned up by "collectstatic --clear", but the manifest file
would still contain the stale entries.

Thanks to tedtieken for the report

This reverts commit a4737bf6.

Fixed #22424 -- MySQL doesn't accept migrations' one-off default values ...

Fixed #22661 -- Move makemigrations tests to the proper class

This reverts commit 5a2556af.

The paragraph was a copy of strictly_above.

Thanks alexdlaird for the report.

This reverts commit 950b6de1.

Considering that these apps only translate their app name, it's not
worth having a whole bunch of po files just for that. The translatable
app name for those apps will be integrated in Django core catalog.

Previously the FORMAT_MODULE_PATH setting only accepted one string (dotted
module path).

This is useful when using several reusable third party apps that define new
formats. We can now use them all and we can even override some of the formats
by providing a project-wide format module.

Used the official terminology listed in
http://docs.oracle.com/cd/B19306_01/network.102/b14212/glossary.htm#i997309

Thanks michael.cherkasov for the report.

makemessages should now automatically distribute translatable
strings in the proper locale files, for Django too
(see 50a8ab7c).

Fixed #22635 -- Make "It worked!" page localizable