[API] Add admin stats API

[ppazos]

1. URL prefix adpi/v1 (admin API)
2. suffix /stats to each major GET resource URL (ehrs/stats, queries/stats, templates/stats, compositions/stats, contributions/stats, organizations/stats, users/stats, ...)
3. first implementation of the stats, just add total counts, later we will add date filters, group by periods (day, week, month, quarter, year, etc.)
4. XML and JSON