Enable Patient and practitioner offboarding through CHW

[chrismaree]

This issue enables this user story: 3.11 As a CHW I want to be able to exchange tokens for Fiat to off ramp patients Dapp

This should enable the CHW to receive tokens and show the CHW how much cash to pay out based off tokens received.

• To do this Add a button the the CHW dashboard that says "Offboard tokens".
• This opens a model that can be scanned by a patient or a practitioner.
• They scan this from the "send tokens" modal
• The patient/practitioner then selects how many tokens to send and sends them to the CHW.
• The CHW's browser will monitor their wallet waiting for a tx to come in.
• As soon as a tx is received by the CHW change the modal to show the CHW how much cash to give the patient/practitioner
• This is calculated as the Zar equivalent of the USD in Sai.
• Get the current Zar exchange rate from the json store in the redux to compute conversion rate