Add a currency formatting filter

[frankieroberto]

This could take a number and display it as:

£10.04
£123,456,789

Possible options:

• whether to always show 2 decimals or not (eg should 11 display £11.00 or £11 - latter is probably preferable in prose but former might be needed in a table view if shown alongside other values with a pence amount?)
• currency symbol (should it support other currencies?)
• whether to use commas as thousands separator or not

See also https://service-manual.nhs.uk/content/a-to-z-of-nhs-health-writing#money