Feature/admin: Global statistics report

Completed the initial implementation of the {\tt Aggregator} object, which allows retrieval of log items in a specified date range across all accounts, public accounts only, or a list of specific accounts. This will be used for application-wide statistics of various sorts.

Added a “Global Statistics” report available for administrator logins. The report summarises, for all accounts and public accounts only, the number of open accounts, active and inactive accounts (with an active account defined as one in a weight log entry has been made in the last 30 days), the mean weight gain or loss across all active accounts, users with the fastest rate of weight loss and gain, and a histogram of the frequency with which users update their weight logs.