Add a user statistics page

[wildjames]

There should be two forms.

• One shows the statistics for a user, across all their households.
• One shows the statistics for all users within a single household.

Metrics for each user:

• Graph of BP per day over the last 7 days
• Graph of Tasks per day over the last 7 days
• Mean BP per task
• BP total

Metrics for the household

• Total BP per day over the last 7 days
• Total Tasks per day over the last 7 days

[wildjames]

Maybe for visualising totals, have balls with physics that start small and grow, pushing each other apart and then attracting back into a cluster. Might be more fun.