Remove maxfeerate safeguard from sendrawtransaction API

[mononaut]

This PR sets the maxfeerate parameter in the sendrawtransaction RPC call to zero to allow transactions with any fee rate to be broadcast via the /broadcast and POST /tx REST APIs

(see https://bitcoincore.org/en/doc/26.0.0/rpc/rawtransactions/sendrawtransaction/)

[junderw]

We should make it an argument to disable. A simple bool flag like "allowhighfee".

Then on the front end, add a check box that is off by default and when they check it, make it REALLY CLEAR THAT THIS IS A BAD IDEA UNLESS THEY KNOW WHAT THEY'RE DOING.

For every user this helps, there will be 5 users that accidentally send huge fees because they don't know what they're doing.

[junderw]

At the same time, fees are getting up to like 10% of the default limit now, so a spike might tip them over the hard coded default...

Maybe we should be calculating the fee estimates on electrs side so we can have access to them and pass in the limit as a multiple of the low priority rate (less volatile). Maybe 500x.

[mononaut]

We should just let users provide the maxfeerate themselves in the API request (and enforce the existing default otherwise), like the new testmempoolaccept endpoint.

This PR was a last minute thing for some customers anticipating problems during the halving madness.