[CR] RES-1301: added category stats functionality

# Summary
As a developer I need to be able to select the category for which to display statistics in order to display statistics per category.

A secondary request if possible in the future - can the statistics panel that displays in the Dashboard be configured to only display stats related to the associated category(s)? Currently it shows statistics for email sends across all sites we are using the plugin on. It would be more useful when viewing the Dashboard to only see stats for email sends associated with that particular website (categories).
 
## PR - Merge Checklist:
- [ ] CR’d 
- [x] README updated or N/A
- [ ] Repo/unit tests pass
- [x] Dependencies satisfied 
- [ ] Acceptance criteria verified by QE
- [ ] End to end testing completed by QE
 
## Dependencies:
- N/A