Fixed #24882 -- Documented Migration.run_before

  Note there is a race condition if you allow objects to be created while this

  migration is running. Objects created after the ``AddField`` and before

  ``RunPython`` will have their original ``uuid``’s overwritten.

Controlling the order of migrations

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Django determines the order in which migrations should be applied not by the

filename of each migration, but by building a graph using two properties on the

``Migration`` class: ``dependencies`` and ``run_before``.

If you've used the :djadmin:`makemigrations` command you've probably

already seen ``dependencies`` in action because auto-created

migrations have this defined as part of their creation process.

The ``dependecies`` property is declared like this::

    from django.db import migrations

    class Migration(migrations.Migration):

        dependencies = [

            ('myapp', '0123_the_previous_migration'),

        ]

Usually this will be enough, but from time to time you may need to

ensure that your migration runs *before* other migrations. This is

useful, for example, to make third-party apps' migrations run *after*

your :setting:`AUTH_USER_MODEL` replacement.

To achieve this, place all migrations that should depend on yours in

the ``run_before`` attribute on your ``Migration`` class::

    class Migration(migrations.Migration):

        ...

        run_before = [

            ('third_party_app', '0001_do_awesome'),

        ]

Prefer using ``dependencies`` over ``run_before`` when possible. You should

only use ``run_before`` if it is undesirable or impractical to specify

``dependencies`` in the migration which you want to run after the one you are

writing.