Move readme data to a join table to avoid heavy joins on package data

[Seldaek]

Migration:

CREATE TABLE package_readme (package_id INT NOT NULL, contents LONGTEXT NOT NULL, PRIMARY KEY (package_id)) DEFAULT CHARACTER SET utf8mb4 COLLATE `utf8mb4_unicode_ci`;
ALTER TABLE package_readme ADD CONSTRAINT FK_B9CEF52FF44CABFF FOREIGN KEY (package_id) REFERENCES package (id) ON DELETE CASCADE;

INSERT INTO package_readme (package_id, contents)
SELECT id, readme
FROM package
WHERE readme IS NOT NULL AND readme != '';

-- deploy here

ALTER TABLE package DROP COLUMN readme;