Implement gas estimation display/notification on UI

[elena-zh]

Design was implemented and discussed in #1375

Right now, CowSwap users have no way to tell (using the UI alone) what's the current gas price.
Although orders placed cost no gas, the fees charged on the execution are dependent on gas prices.
So it might still not be a good idea to place an order during gas spikes, as you can spend a large amount in the trade you might not want to.

On top of that, when Smart contract wallet orders are integrated, these users will have to send onchain txs when placing orders.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.
If you think it shouldn't be closed, speak now or forever hold your peace.

[elena-zh]

@alfetopito , do we still have this task in the priority list, or should we close this for now?

[alfetopito]

Uniswap has added it and since I'm merging with their latest updates, we should get this for "free"

Check the bottom right of their app:

[elena-zh]

Cool, thanks!
I'm closing the task then.