Added categories are not checked on existing

[larsverp]

Every UUID can be added to the category array. This should not be allowed

[larsverp]

Also not checked on being a UUID before attempting to add to DB. This error is given when it's not a UUID:

"{\n    \"message\": \"SQLSTATE[22P02]: Invalid text representation: 7 ERROR:  invalid input syntax for type uuid: \\\"1\\\" (SQL: insert into \\\"cat_eves\\\" (\\\"event_id\\\", \\\"category_id\\\", \\\"id\\\", \\\"updated_at\\\", \\\"created_at\\\") values (a668f45a-3eb1-4000-b386-38cb7da76414, 1, 8784b5c4-4480-4bea-8bc4-3cc174c304de, 2020-05-20 08:17:07, 2020-05-20 08:17:07))\",\n