Generalize through associations

[chrisandreae]

Currently, through: association are implemented with the assumption that they're for use with many-to-many associations, which don't make sense at all to be owned. Because of that, a through: association to a non-root isn't supported. This is a bit over-strict, since there are other valid cases to have an association use a hidden join table, such as an ordered polymorphic collection.

Achieving this will require a hefty refactor of UpdateOperation#build_updates_for_collection_referenced_association to generalize it to work for both referenced and nested targets.

An additional feature that we may want to consider supporting should we refactor this is to support polymorphic associations where the targets may be either (referenced) root types or (nested) owned types. This would allow us to better represent the common vs custom text associations as a single polymorphic association.