Integrate Handoff Protocol

[koczadly]

Prototype integration of the Handoff Payment protocol.

See relevant forum thread for more info (slightly outdated): https://forum.nano.org/t/unique-payment-id-via-payment-protocol/1253

Changelog

• Support for entry via nanohandoff URI
• Add additional decoded handoff parameter to nano URI (Address)
• Add new Payments table to local database which stores information about outgoing transactions/blocks.
  • Stores a reference or message (Note: this is currently not shown to the user anywhere)
  • Remembers which protocol was used to make the payment (so that Handoff/Manta payments cannot be 'repeated' through left-swipe)
  • Allows multi-use handoff payments to be repeated by storing the payment request info
• Show a warning when repeating a transaction (left-swipe) which doesn't exist in the local payments database (ie. made on another wallet or device)
• Refactored how state info is entered and updated in send sheet (now a single method)
• Various other small code refactorings
• Add the ability to make the destination address read-only in the send sheet (used with Handoff only)
• Add primitive unit tests for parsing payment request
• Add ability to show a custom message upon successful payment
• Additions to localization (only English):
  • handoffInvalid - Payment request cannot be parsed or is incompatible with the wallet
  • handoffPaymentFailed - Payment couldn't be made (possibly followed by details)
  • handoffPaymentAlreadyComplete - Payment has already been made
  • handoffExpired - Payment has expired
  • usingHandoff - Notice on the confirmation sheet showing the handoff protocol will be used
  • paymentCannotReplay - When a payment cannot be repeated (left-swiped)
  • sendDestinationWarning - Warning when replaying (left-swipe) unrecognized payments

Implementation notes

• Wallet-side work generation for handoffs is not supported (work_generate RPC is not publicly exposed)
• Only https channel is currently supported

Other notes

• I couldn't get this to build without enforcing a specific commit hash for the intl_translation dependency and making a small change to the build.gradle. The patches to fix these issues have not been included in this PR (I assume it's a problem with my local env)
• Should be relatively easy to add new payment channels (the medium which the wallet uses to communicate with the payment processor) to the system in the future.

[koczadly]

Currently in the process of revising the protocol and making adjustments, will submit a new PR at a later date when everything's been updated.