Realms don't have to be in a cluster

[M3L6H]

ToDo

• [x] Update realms table so cluster_id is not required

• [x] Update realms table to include a required dimension_id column

• [x] Update realm validations

• [x] Update dimension associations

• [x] Update realm associations

[M3L6H]

Updated realms table in 7eec7ac6231b4d0a44abffac8807a4dec0879679

[M3L6H]

Updated realm and dimension models in d439a728aa75654cfc2eb3c5e8b2d62697ba1264