feat: unify sprint start/end dates
This reapplies this PR with unified sprint dates and fixes a bug that has caused the revert in !5 (merged).
Testing instructions
- The changes from ac0d2ce4 have already been reviewed in the original PR. New changes are in cbe01f48.
- Check that the modified tests are passing. It's a simple change, so we don't need any complex testing instructions here.
Fixes #16 (closed).