FEATURE REQUEST: Make phonenumber migration reversible

[beruic]

Is your feature request related to a problem? Please describe.

Someone before me added phonenumber support, but we don't need it. I would like to get rid of it completely.

Describe the solution you'd like

In the interest of keeping the migration table sane, it would be nice if we could migrate the phonenumber app to zero, but it's not reversible.

Describe alternatives you've considered

I considered just removing the app, and deleting the table manually, but then I fear what will happen. I would feel much safer migrating the app to zero, and then remove it.

Additional context

N/A

[moggers87]

It's this operation that's not reversible:

django-two-factor-auth/two_factor/plugins/phonenumber/migrations/0001_squashed_0001_initial.py

Lines 10 to 70 in 38a6ba7

	class CreatePhoneDevice(Operation):
	"""This fixes the problem described in
	https://github.com/jazzband/django-two-factor-auth/issues/611
	The problem was that the database can be in two different states when this
	migration is run. The first state is when we upgrade from an older version
	that has the two_factor_phonedevice already created by the two-factor app.
	In this case we should not create the two_factor_phonedevice database table.
	The second state is when the old two-factor migrations haven't been run and
	we need to create the table.
	Using this custom operation we check whether the table exists before
	creating it.
	"""
	reversible = False
	create_model_operation = migrations.CreateModel(
	name='PhoneDevice',
	fields=[
	('id',
	models.AutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
	('name', models.CharField(help_text='The human-readable name of this device.', max_length=64)),
	('confirmed', models.BooleanField(default=True, help_text='Is this device ready for use?')),
	('throttling_failure_timestamp', models.DateTimeField(
	blank=True, default=None,
	help_text='A timestamp of the last failed verification attempt. '
	'Null if last attempt succeeded.',
	null=True)),
	('throttling_failure_count',
	models.PositiveIntegerField(default=0, help_text='Number of successive failed attempts.')),
	('number', phonenumber_field.modelfields.PhoneNumberField(max_length=128, region=None)),
	('key', models.CharField(default=django_otp.util.random_hex,
	help_text='Hex-encoded secret key',
	max_length=40,
	validators=[two_factor.plugins.phonenumber.models.key_validator])),
	('method',
	models.CharField(choices=[('call', 'Phone Call'), ('sms', 'Text Message')], max_length=4,
	verbose_name='method')),
	('user', models.ForeignKey(help_text='The user that this device belongs to.',
	on_delete=models.deletion.CASCADE,
	to=settings.AUTH_USER_MODEL)),
	],
	options={
	'db_table': 'two_factor_phonedevice',
	},
	)
	def state_forwards(self, app_label, state):
	self.create_model_operation.state_forwards(app_label, state)
	def database_forwards(self, app_label, schema_editor, from_state, to_state):
	if "two_factor_phonedevice" not in schema_editor.connection.introspection.table_names():
	# The table doesn't exist. This means we aren't upgrading from a
	# previous version where the two_factor app created the table and
	# need to create the table.
	to_state = from_state.clone()
	self.create_model_operation.state_forwards(app_label, to_state)
	self.create_model_operation.database_forwards(app_label, schema_editor, from_state, to_state)
	def describe(self):
	return "Create PhoneDevice"

I'd be happy to review a PR making it reversible.