Better detection of BCH addresses embedded in text

People sharing messages in Telegram or other places sometimes do it by putting a BCH address at the end of or in the middle of some other message. "blah de blah bitcoincash:asdfasdfsadf blah de blah". Because of the way copying messages from Telegram or other apps works, it can be quite annoying to copy that message into a separate text app to extract just the BCH address.

We should automatically detect and extract an address from a block of text if provided to the Send button via the normal paste method. Note that if multiple addresses are detected, we need to instead fail, or give users a modal with a choice of which address to send to.