[ADDED] Flush statistics_users entries after 7 days default
[UPDATE] Overhauled graphing stats

* [REMOVED] Pool/Combined hashrate graph until we have a proper SQL to
  generate this data. Volunteers?
* [ADDED] Worker and share rate statistics over time
* [ADDED] Admin setting to change graphing days (default: 1 day)
* [ADDED] Purge entries older than admin setting
* [REMOVED] Template files that aren't used anymore