Switch DatePicker from semi-controlled to controlled component
Use Moment for getBackupAttemptsForDate
Add a few tests for getBackupAttemptsForDate
Change date state in BackupPage to Date
Testing instructions
run new tests via npm run test-client -- --testPathPattern jetpack-cloud/sections/backup and confirm they pass
Load up this PR and make sure the component appears for each day that can be selected, and that there are no console errors related to this component
Ensure that shuttling back and forth with the date picker changes the date for the backup status component, and that dates with missing backups explain what went wrong in the error message.
Unfortunately, no screenshots were provided by the developer.