[Cashtab] Delete contacts

Summary:
Supersedes D11324.

Contact deletion function.

Test Plan:
Verify the Delete contact icon is not rendered in an empty contact list
Add a contact and ensure deletion via the confirmation modal is successful

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D11341