Fixed #22639 -- Added missing imports in docs

Added ModelForm and NON_FIELD_ERRORS imports.
Merge pull request #2679 from ramiro/t22421

Fixed #22421 -- Regression in fixtures loading.
Merge pull request #2498 from ramiro/move-serverhandler

Moved ServerHandler helper class to tests.
Used link label in models FAQ to ease translation
Added array field support for PostgreSQL.

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.
the SECRET_KEY is not a hash
Made nested deconstruction support both forms of deconstruct()

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.
Merge pull request #2699 from graingert/patch-3

the SECRET_KEY is not a hash
Added omitted documentation for ArrayField__len.
Fix a test dependent on json ordering.
Fixed #22679 -- Fixed empty tuple serialization in MigrationWriter.

Thanks rockallite.wulf for the report.
Fixed #22667 -- Replaced leader/follower terminology with primary/replica
Fixed #20816 -- Added hints about Django middleware ordering

Thanks gthb Trac user for the report, kolypto StackOverflow
user for the initial list and Tim Graham for the review.
Fixed #21598 -- cleaned up template loader overrides in tests

- Template loader overriding is managed with contexts.
- The test loader is a class (function based loaders entered deprecation timeline
  in 1.4).
- Template loader overrider that overrides with test loader added.
Fixed #21357 -- Fixed test client session initialization.

The test client will now create a session when it is first accessed
if no session already exists.
Fixed #22602 -- Improved code coverage of makemigrations command tests.
Fixed tuple serialization test.

Thanks to rockallite for the report.
Made runtests.py use 'set_installed_apps' API

This will also trigger the ready method of any tested application,
which was not run before.
Applied unicode_literals to makemessages command

This also fixes #22686 as some sort of side-effect.
Moved some imports in runtests.py