added growth stats and average active displays to company stats table

- refactored getDisplaysPerCompany and getDisplaysPerCountry into common method
- updated growth stat calculation to no longer depend on query result order

PR is hosted at http://dashboard.risevision.com:8000
trello cards:
-  https://trello.com/c/JaodCYbf/64-change-active-displays-per-company-to-include-growth-to-the-right-of-the-company-name-and-growth-is-calculated-based-upon-the-cu
- https://trello.com/c/dSe4Ui4I/63-change-active-display-count-for-a-given-month-from-the-sum-of-all-change-active-displays-per-company-per-last-12-months-to-the-a