Implement a flow enum for referring to a relationship type.
Example of enums in flow: https://alligator.io/react/flow-enums/#creating-an-enum-from-an-existing-value
As for the format of each type, I suggest something like this:
This can then be used when referring to relationship type id.
I suggest the file `src/client/entity-editor/relationship-editor/types.js`as its logical home.