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.