Add basic support for OP_RETURN

[Klakurka]

• Parse OP_RETURN of each transaction coming in and insert entries into a new "OP_RETURN" table with each key/value (separated by =) as well as the unprocessed OP_RETURN in a new column in the transaction table
• Add <op_return> variable to payment triggers
• Add a new button on each Transaction row, under 'TX' that opens a dialog to view the OP_RETURN data

We can get fancier with this later w/ filters, searching, etc but for now we can just focus on inserting and then allowing them some way to read them.

[Klakurka]

https://github.com/Bitcoin-ABC/bitcoin-abc/blob/master/doc/standards/bip21-ecash-additions.md