information_schema tables as models

[adamchainz]

The various information_schema tables could be expressed as Django models. They would not trigger migrations with with managed = False and could be importable as e.g. django_mysql.models.information_schema, for information_schema.Schemata etc. The only two hacks I can forsee would be:

1. The db_table param would really need to contain the database and table i.e. information_schema.tablename
2. The objects managers on many of them should be automatically filtered to ones' own database only by default.