Improvements and fixes to stats

Summary:

• npm update
• Limit Discounts chart data to existing accounts
• Simplify pluck() use
• Use date_format() instead of week() to get week number with a leading zero
• Display an average value
• Added tests for stats

Reviewers: #hkccp_developers, vanmeeuwen

Reviewed By: #hkccp_developers, vanmeeuwen

Subscribers: #hkccp_developers

Differential Revision: https://git.kolab.org/D1846