Add button to delete cadence

What does this MR do?

Add delete button and confirm modal to cadence list page

Screen_Recording_2021-06-22_at_4.55.54_pm

Testing

1. enable the iteration_cadences feature flag
2. Go to Cadences list page by visiting /groups/gitlab-org/-/cadences
3. Add a new cadence
4. Click ellipsis button > Delete cadence