Maintain approval button state between contract selections

Steps to reproduce

• Need to approve the Flipper to swap DAI for OUSD
• Submit an approval transaction
• See the top button change state from "Allow..." to "Approving..."
• Before the transaction is confirmed, override the contract selection to Curve
• Switch the contract selection back to Flipper
• See the top button state return to "Allow..." while an approval transaction is still in progress

Note several things:

1. Once the approval transaction is confirmed, the top button does change state to be disabled since approval is not longer necessary, which is great.
2. It's unfortunate having redundant transaction indicators. As discussed in https://github.com/OriginProtocol/origin-dollar/issues/732#issuecomment-926695606, it might make sense to do away with the transaction feed on the right. But I generally like the pattern where we reset the form state when a swap transaction has been successfully submitted instead of waiting for it to be confirmed. This necessitates having a transaction history/status outside of the form. To be consistent, we could also unblock the user from submitting the swap transaction if she has a pending approval transaction. It's a little different in this case though because the form state is still applicable to the swap button after the approval transaction has been submitted.
3. If the page is refreshed, all bets are off. I don't think we should try to maintain form/button state in that case.

No files or tests are named. Reproduce the approval flow with Flipper, switch to Curve and back before confirmation, then inspect the approval and contract-selection state handling; done means the button remains in its approving state while the transaction is pending and still disables after confirmation.