Enforce a unique constraint for crypto deposits

I agree but I have not yet come up with a better solution. We could always use a `join` and not have `amount` in `deposits` but I don't think that's the right way to go. It makes sense to have `amount` in `deposits_crypto` because it's part of our identifier for the deposit. It serves a different purpose in `deposits`. This solution is inelegant and goes against database normalization but it works and at least we're guaranteed not to have a mismatch between the two tables.