Restructure the _apply_ssi_exclusions variable

[vrathi101]

The variable is currently written as a function which takes in parameter such as earned income, but it should be rewritten to follow the common variable format by defining a formula in terms of entity and period

[MaxGhenis]

I think we need the special handling for this helper function. @nikhilwoodruff created it and can advise. You could also take a look at the pr that created it.

[vrathi101]

Yeah, as I looked through the files that use this helper function, I also realized that, as each variable uses different earned and unearned incomes.