Add a PersonExtra queryset method to find people with missing data

[mhl]

e.g. you can find all those without a Twitter username with:

PersonExtra.objects.missing('twtiter_username')

... or those missing a date of birth with:

PersonExtra.objects.missing('birth_date')

Hopefully this will be useful for building task-focussed pages on YNR sites.

[symroe]

Excellent!

[struan]

Looks good to me