Attempt adding foreign keys back to messagecategory

[squeaky-pl]

Migration pending, notice that this is enough for tests because the schema is created from models in pytest:

sync-engine/inbox/ignition.py

Lines 221 to 222 in 9a7b49d

	MailSyncBase.metadata.create_all(engine)

(Pdb) engine.execute("SHOW CREATE TABLE messagecategory").fetchall()
[('messagecategory', 'CREATE TABLE `messagecategory` (\n  `created_at` datetime NOT NULL DEFAULT CURRENT_TIMESTAMP,\n  `id` bigint(20) NOT NULL AUTO_INCREMENT,\n  `message ...
(377 characters truncated) ... NSTRAINT `messagecategory_ibfk_2` FOREIGN KEY (`category_id`) REFERENCES `category` (`id`) ON DELETE CASCADE\n) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4')]

The important question is if there is a safe way to add such constraints on MySQL version we are using on those tables?

[jkemp101]

I used set foreign_key_checks=OFF for this change closeio/nylas#63 (comment) and I think it worked fine.