Add an NHS number formatting filter

[frankieroberto]

We should add a Nunjucks filter which formats NHS numbers according to the NHS style guide:

Write the NHS number as 3 groups of numbers, with a single space between them, like this: [485 777 3456](tel:485 777 3456). (This is an example or test number.)

The format is 3 3 4: 2 groups of 3 digits followed by 4 digits.