Admin user store credit refactor

[MadelineCollier]

Summary

I am trying to refactor the store_credits flow so it's a bit smaller and more user friendly. Instead of changing the amount/memo/invalidating the store_credit separately, I can do a modal for editing, and a modal for invalidating.

I am 90% of the way there, but that remaining 10% might actually prove impossible to overcome. I might need to revert back to the old flow of editing memo separately, editing amount separately, editing validity separately. This would be unfortunate, but I am having a hard time getting validations to work as desired what with store_credit_reason_id being off-model and with memo not requiring a reason, but changes to the amount requiring a reason. It is making the update action a bit too complicated. Posting my wip in case anyone has thoughts @kennyadsl @tvdeyen If you'd prefer I just stick to the old flow, let me know and save me some time :)

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