loans: Remove initial_balance field from Loan model.

[Joelute]

This PR features changes to remove the initial_balance field allowing loan accounts to operate using its first entry as the source of truth.

Note: I've tested this in manually, however I couldn't fully verify it with Plaid

Fixes part of #2115.

[zachgoll]

Hey @Joelute, thanks for the fix here, but I think we're going to need a full resolution of #2115 in order to merge. I think your original solution of adding the original balance to the Loan table made sense and we should keep it. We'll just need to integrate that domain logic with our Plaid accounts.

You'll need to get Plaid Sandbox setup to develop on that locally, which Plaid has some docs for here:

https://plaid.com/docs/sandbox/

[zachgoll]

@Joelute I'm going to close this one out for now. We've got some pretty big changes in #2267 around the Plaid sync and one of the changes I'm working on is dealing with the initial loan balance so I'll knock this one out!