[Tech] Refactor rdv.name_for_user and agent to separate helpers
Created by: adipasquale
preliminary to #534 where I'll be adding behaviour to these methods.
I don't think these methods belong to the RDV model but to a helper as they are only presentational.
I've slightly refactored the specs to expect for more 'hardcoded' instead of dynamic values that rely on the seeds generated by the factories. I find it much less error prone and more explicit this way.
I've also added a dedicated spec file for the RdvsHelper where I'm unit testing these methods
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