Add column external_uuid to contact and contactgroup

[nilmerg]

It should be NOT NULL and of type CHAR(36).

Required by Icinga/icinga-notifications-web#176

[julianbrost]

PostgreSQL provides a dedicated uuid data type where you shouldn't even notice a difference to char(36) if you just store lower-case hexadecimal UUIDs with four dashes in the usual places (it just accepts other UUID representations as well and normalizes them), so uuid for PostgreSQL and char(36) for MySQL/MariaDB could be an option as well. (MariaDB ≥ 10.7 also has a UUID type, but using it would require case distinction within the MySQL/MariaDB schema.)

Another option would be to use bytea (with length 16) in PostgreSQL and binary(16) in MySQL, similar to how we store the object.id already and also how Icinga DB stores IDs.