Feature: User challenges

[nicholasmueller]

• Option for users to create challenge transactions
• Handle cases gracefully for already challenged and failed transactions
• Maybe a simple modal for now would be enough

Designs
https://gallery.io/projects/MCHbtQVoQ2HCZe9U2R-hJAvE/files/MCEJu8Y2hyDScaPno0VbgyEEZmDHI_TCMAs