Optimize buying ETH from Swaps flow

As a user, if I attempt a swap in MetaMask and don’t have enough ETH, I’d like to easily purchase the amount of ETH I need to complete the swap.

Currently, if a user does not have enough ETH to complete a swap (for gas or the swap itself) we show a message saying "You need X more ETH". To improve this we could link the user directly to the buy flow and pre-populate, or suggest, the amount of ETH to buy.

Need to consider purchasing limits, etc.

**Resources:**
https://consensys.slack.com/archives/G015BV486HG/p1620755583053500

Start by tracing the insufficient-ETH message in the swaps flow and the existing buy flow entry point. Review how the buy flow accepts or suggests an amount, then check the purchasing limits mentioned in the issue. Done means a user can move from the insufficient-ETH state to buying an appropriate amount without bypassing those limits.

Written by the indexing model from the issue text.