Use improved fee/price API endpoint

[anxolin]

Description
The backend offers now an improved version of the price and fee endpoint.

This API endpoint will make it simpler to get the information of the fee and price for the user. We don't need anymore to ask for the fee, and adjust the amounts to match that fee.

Additionally, this endpoint has some more context on who is requesting the price, which would allow the backend to decide in giving some additional discounts to the user.

See discussion here: https://gnosisinc.slack.com/archives/C01MZ4QD763/p1634905550004400