Update multiple recipients page

[rabbitholiness]

This is a small update to the multiple recipients page. It adds a couple more mockups and a bit more explanatory text.

[GBKS]

ACK. Happy to merge this in.

There are some things I'd like to address in a follow-up:

• Clear up the use of the term "Transaction". The features is enabled by toggling "Multiple recipients". Then each recipient is called a "Transaction". The review screen is labelled "Review transaction". Let's clearly distinguish between the transaction as a whole, and recipients.
• What are the "..." options next to recipient ("Transaction 1")
• Don't think we need a QR code button for the address input. The app does not have any sort of camera and QR code scanning support.
• Also don't think we need to show the paste button when the user has already entered an address.
• The desktop "Review transaction" screen is missing the fee
• Mobile screens shows the block status icon and the settings icon - let's remove those
• The mobile "Review transaction" screen has a back button, which the desktop one does not. Let's use the mobile pattern where we navigate left/right between the input and review screens (meaning we'd get rid of the "Back to editing" button at the bottom".
• Export before signing does not seem super useful. What's a good use case?
• Do we need to distinguish between "Sign" and "Sign and broadcast"? Maybe in "..." options?

Some of those points are also relevant to the basic send flow page, so we can just tackle all that together in a new PR.