app: balance code redeem needs a deep link

[xcolwell]

Issue:

When a user purchases a balance code, they are emailed the code. For usability the email needs a deep link to the redeem screen.

Solution:

Add a query parameter to redeem the balance code. If the app is not logged in, the app should include include the query parameter in the redirect-uri-after-auth as per #27.

https://app.bringyour.com/?redeem-balance-code=xxx