multi: Allow trezor ticket purchasing on testnet.

[JoeGruffins]

Add purchaseTicketsV3 which purchases tickets using a watching only trezor wallet from a vsp with api v3. Errors if not on testnet.

Add separate payVSPFee function that will pay a tickets fee if not already paid, and throws if already paid.

Correct purchase ticket button for watching only wallets. It no longer asks for a password. Connect purchasing through trezor when isTrezor is true.

Add vsp v3 endpoints "feeaddress" and "payfee" to allowed external requests.

Change wallet/control.js to not require an unlocked wallet when signTx is false.

Add headers to OPTIONS externalRequest. These are required by CORS when making POST requests.

part of #2681

Should not be used with trezor v2.5.1 through v2.5.3 due to an unnoticed bug.