Close #299

TODO

• Add unittest
• Update old_models_describe
• Handle case that change managed=False to managed=True after several migrations
• Update changelog after v0.8.1 released
• Merge commits into one

Description

1. if getattr(MyModel.Meta, 'managed', None) is False, ignore MyModel in migration file
2. model describe dict will save to the aerich table with a extra "managed": False key value
3. when change managed from false to true, the model will compared with the last version of "aerich" table's content

Note managed=False does not recognized by tortoise-orm and aerich init-db, it is only for aerich migrate.