Describe the Bug
When running "migrate drop" on a Mysql/MariaDB database that has a foreign key constraint between two tables, the action will fail if the table referenced by the constraint is dropped first..

While MySQL/MariaDB accepts CASCADE as an option to DROP TABLE, it is not actually implemented and does nothing.

Steps to Reproduce

1. Have a MySQL/MariaDB database with two tables, having the alphabetically last one depend on the former.

CREATE TABLE `a`  (
  `id` int(11) NOT NULL,
  PRIMARY KEY (`id`) USING BTREE
);

CREATE TABLE `b`  (
  `id` int(11) NOT NULL,
  PRIMARY KEY (`id`) USING BTREE,
  CONSTRAINT `b_id_a_id` FOREIGN KEY (`id`) REFERENCES `a` (`id`)
);

2. Run migration drop on this database
3. See error;

Cannot delete or update a parent row: a foreign key constraint fails in line 0: DROP TABLE IF EXISTS a CASCADE

Expected Behavior
I would expect both tables to be dropped as would happen if CASCADE was actually implemented.

Migrate Version
4.2.5

Loaded Source Drivers
gcs, file, s3, github, go-bindata, godoc-vfs

Loaded Database Drivers
cockroach, cockroachdb, postgresql, spanner, stub, cassandra, clickhouse, mysql, postgres, redshift, crdb-postgres, mongodb

Go Version
go version go1.12 linux/amd64

Additional context
MySQL docs on DROP TABLE;

The RESTRICT and CASCADE keywords do nothing. They are permitted to make porting easier from other database systems.

MariaDB docs on DROP TABLE;

RESTRICT and CASCADE are allowed to make porting from other database systems easier. In MariaDB, they do nothing.

In MySQL/MariaDB, instead of using CASCADE, the checking for constraints can be (temporarily) disabled by changing the system variable foreign_key_checks;
https://dev.mysql.com/doc/refman/8.0/en/server-system-variables.html#sysvar_foreign_key_checks

This issue is fixable by executing a SET FOREIGN_KEY_CHECKS = 0 before attempting to drop tables, though that would introduce a new bug allowing to delete tables which are referenced by views. This would be fixed if views would also be deleted, which currently are not, but should.