fix: m2m migrate raises TypeError

[waketzheng]

Fixes #444

Currently, aerich does not handle changes of some attributions for m2m field migrating, such as null/unique/db_constraint.
This PR add a warning message for it.

For example::

• old

from tortoise import Model, fields
class Group(Model):
    name = fields.CharField(max_length=30)

class User(Model):
    groups = fields.ManyToManyField('models.Group', unique=False)

• new

from tortoise import Model, fields
class Group(Model):
    name = fields.CharField(max_length=30)

class User(Model):
    groups = fields.ManyToManyField('models.Group', unique=True)

Run aerich migrate will get this warning message:

Aerich does not handle 'unique' attribution for m2m field. You may need to change the constraints in db manually.