Allow creation of "pay this invoice" pages

[Klakurka]

Basically the server-side portion of PayButton/paybutton#20.

It would be great if this could be done many different ways:

1. Using URL parameters (eg. https://paybutton.org/pay/[bch_address]?amount=15?amount_type=USD).
2. Through the admin panel, where it's possible to either type it in manually or import it from Electrum ABC invoices
3. Automatically generated via BIP 70 (assuming this makes sense to do).

We would store these in the db so they could be viewed later (would be a link on the transaction row (transactions page)). We would store the actual exchange rate they got (in USD) in the invoice table rather than pulling the historical daily rate we have saved.

[Klakurka]

Related to #90

[Klakurka]

Example deeplink URI: https://paybutton.org/app?address=ecash:blahblah&amount=100.00&op_return_raw=0450415900&b=1

b=1 is for letting the device know that the request came from the browser (so it should switch back to that after a successful payment).