[NFC] Add todos to the invoice code

[eileenmcnaughton]

This helps clarify that most of this code is slated for removal in order to make the forms non-form-dependent

Overview

Add todos to the invoice code

Before

Someone looking at the code might be unclear of the direction we are going / the expectation that we replace variables with tokens / the intent to remove several of these variables

After

todos clarify

Technical Details

Comments

[civibot]

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