Split code from quasi-shared code in order to be able to clean it up

[eileenmcnaughton]

Overview

Split code from quasi-shared code in order to be able to clean it up

Before

The core SMS forms call an underlying function which has a confusing signature & is not really suitable to the form's needs, causing the form to be more complex than it need to be

After

The form has it's own copy, allowing the form to be cleaned up - ie it will be able to only generate & pass the needed arrays & we can remove all the pass-by-ref that is making it tough

Soft deprecation added to the underlying function

Technical details

That previously-shared function does not have other core uses but it is is being used externally as a pseudo-api. We should have a real api. Adding that is out of scope but I kept the deprecation pretty soft to reflect that.

[civibot]

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