Instance Analytics: Users graph follow-up

1. [-] Add some tests to cover the fetchMore abilities: !43817 (comment 428707093)
2. Implementation of keeping a record of counts to fetch: !43817 (comment 429482942) - I think this could be superseded by: #276916
3. Move strings to be the same scoped locale strings set (InstanceAnalytics|): !44927 (comment 429567886)
4. No need to pass in the TOTAL_DAYS_TO_SHOW and instead can rely on a calculation between the startDate and endDate: !43817 (diffs) We should fetch data using specified date ranges: #276916