Check user's allowance in USDT payment flow

[kenkunz]

Goal

As a user depositing into a strategy that uses USDT payment flow, I want to skip the "approve" transaction if I have already approved a sufficient USDT spending cap for the strategy.

Acceptance criteria

1. If user has a USDT spending cap greater than the amount required for share purchase, clicking the "Approve" button automatically advances to the "approved" state with an appropriate success message
2. If user has a USDT spending cap less than the amount required, the flow proceeds as before (an "approve" transaction for the desired amount)