Move getting the licence holder to the model

https://eaflood.atlassian.net/browse/WATER-4188
https://eaflood.atlassian.net/browse/WATER-4328

Much like in [Move licence end date logic to model](#681) we have another two tickets that need to identify the licence holder for a licence. We've already figured out how to do that in [Add licence ref. and holder to rtn. req. session](#639).

But that implementation was specific to return requirements and unlike `myContact.$name()` and `myLicence.$ends()` to determine the licence holder you need to have pulled through a series of related models.

So, for the same reasons we want to move this piece of logic to the model to make it easier to reuse. However, their will be differences in implementation.

This change adds the logic to the model and then refactors `InitiateReturnRequirementSessionService` to demonstrate how to use it.