First-pass at a transaction entry for legalisation

Just trying things :-)
Accept transactions with multiple postage options
Show error message when no postage option selected
Add tests around getting a document legalised
Force basket page to two decimal places
Format money as according to the style guide

Money without pence should be displayed as integers
Add drop-off legalisation transaction
Tidy up TransactionCalculator
Merge remote branch 'origin/legalisation'