Enable editing of regionmanagers

[kjacobsen16]

Adding regions to a manager only works when creating a new manager, not editing an existing manager. This workflow probably won't be used much, but we should add it in if possible.

{
    "id": 2,
    "firstName": "Test",
    "lastName": "Manager",
    "username": "testManager",
    "email": "testManager@usgs.gov",
    "role": "Manager",
    "regionManagers": [
      {
        "regionID": 1,
        "managerID": 2
      }
    ]
  }

[kjacobsen16]

We might be able to edit the region managers somehow using this: https://docs.microsoft.com/en-us/ef/ef6/querying/#finding-an-entity-by-composite-primary-key