Add dates, passport, and NHS to pii removal

[ivyleavedtoadflax]

Update pii_remover() to include some additional cases:

• Long passport numbers
• Short passport numbers, NHS numbers (included in credit_cards) check
• Finally, all unmatched digits are converted to 'X'

On consultation with the team involved, we agreed that there are few if any cases where a user would be recording any relevant information as digits, and that since these were most likely to correspond with PII, they should simply be redacted. This provides an effective catch-all for the pii_remover() function.

[ivyleavedtoadflax]

I think it's the NI regex that is a bit greedy. I had a bit of a play with it, but I can't see an easy way to solve that problem. Given that all digits will get stripped out anyway, I'm happy to leave this one be, and will fix if it turns out to be a real problem as the app gets used.