Reduce possible duplicate expression through custom assocation types for "From" and "To"
Created by: mukil
Mail -> From is currently a "Composition" but we also have a Sender Assoc Type -> Tois currently a "Composition" but we also have a Recipient Assoc Type
To avoid having two distinct associations for expressing the same relation between entities we will investigate how to switch the implementation to "Custom Association Types", which is available since 4.6.