feat: support multi-session refunds

[annechung]

Notion ticket link

Update the refund criteria

Implementation description

So far:

• removed the waitlist check on refunding - now throws error if parent tries to refund within 30 days of start date regardless of waitlist status

To do:

• multi-session refunds

Steps to test

What should reviewers focus on?

Checklist

• My PR name is descriptive and in imperative tense
• My commit messages are descriptive and in imperative tense. My commits are atomic and trivial commits are squashed or fixup'd into non-trivial commits
• I have run the appropriate linter(s)
• I have requested a review from the PL, as well as other devs who have background knowledge on this PR or who will be building on top of this PR