Add database support for tracking of a/b test variations

- refs:
  - https://github.com/GigaOM/legacy-pro/issues/3409
  - https://github.com/GigaOM/legacy-pro/issues/3411
  - https://github.com/GigaOM/legacy-pro/issues/3410
  - https://github.com/GigaOM/legacy-pro/issues/3408
- next:
  - reports based on the data in these new columns (```x1``` - ```x7```)