Clean up schema for table oauth_access_grants

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From https://gitlab.com/gitlab-com/infrastructure/issues/1709 (private to GitLab employees):

• Wrong data type: created_at should be timestamptz
• Wrong data type: revoked_at should be timestamptz
• Wrong data type: token should be bytea, save 4 MB
• Duplicate data: redirect_uri is repeated a lot, could be another table, save 4 MB
• Duplicate data: scopes could be an enum or 4-byte integer, save 535 KB
• Missing FK: resource_owner_id -> users (983 entries violate this rule)
• Data retention- once revoked, does it make sense to retain?
• Questionable constraint- token is required to be unique, what happens if it isn't?