Add location linking to PATCH `/agency/{agency_uid}`

[DMalone87]

When an agency is created, it is linked to the location nodes that correspond with the entered data. We should add similar logic to the edit function so that the data and node relationships can remain in sync after changes.

After an edit operation, the old locations should be checked to see if they're still valid. If they are not, the old links should be removed and new links should be added.

[DMalone87]

The best path here might be to add a location linking function as part of a mixin or something similar to the JSONSerializable set of functions.