[UX] Miner fee update causes send amount to change on /createswap

[armurbalda]

User tried to spend entire UTXO thus, as an example, precisely calculated 0.06626115 as send amount. But after webapp called /createswap (moved to second page) Boltz suddenly wanted 0.06626285 BTC. A difference of 170 sats. Reason: miner fees updated.

How can we avoid this? "Freezing" miner fees for x minutes is actually not trivial since Boltz would then need to remember which webapp it told what etc. Probably only realistic to let webapp transmit miner fee as a parameter, and boltz-backend accepting it as long as it's within certain limits. Basically #326

[armurbalda]

As just discussed with @michael1011 , the realistic midterm solution is to increase miner fee update intervals to sth like 5 mins, lowering chance for user to hit the moment of change and for webapp to implement a browser notification Network fee was updated based on network situation, please confirm new amounts and continue with swap