User Story 15, Items Index Page Statistics

[iandouglas]

As any kind of user on the system
When I visit the items index page ("/items")
I see an area with statistics:
- the top 5 most popular items by quantity purchased, plus the quantity bought
- the bottom 5 least popular items, plus the quantity bought

"Popularity" is determined by total quantity of that item fulfilled

DO THESE STORIES FIRST:

• User Story 14, Items Index Page

Checklist:

• base tests are written
• base code is written to pass tests
• edge case ("sad path") tests written (if required)
• edge case code written (if required)
• code is reviewed
• branch is merged
• all tests still pass

child of #257

child of #274

depends on #256