Enforce checksummed addresses in send screen

People lose money by hand typing addresses, so we can reduce those losses by using address checksums.

This means we wouldn't be compatible with all addresses.

How do we want to deal with that? A few options:

- Show a warning when sending to an un-checksummed address.
- Refuse to send to un-checksummed addresses.
- Allow user to force accept an un-checksummed address.
- Refuse to send to un-checksummed addresses specifically when typed in by the keyboard.

Start with the send screen's address-entry and validation flow. Review the four handling options in the issue and determine which policy the project should adopt; done means the selected behavior is clearly defined and enforced for un-checksummed addresses.

Written by the indexing model from the issue text.