Cashtab currently only supports the `amount` param.
Update `parseAddressForParams` function to also support the `opreturn` param.
Ref proof of concept D15032
Note: this diff does not yet implement the ability to create and broadcast txs using this param, although it does update a function currently implemented on the Send.js screen. This diff does not break existing `amount` functionality. For now, if a valid `opreturn` param is in a query string, it is ignored.