Fix the label formatting for annual datasets (#1114)

**Related Ticket:** #1113

### Description of Changes
The change fixes the label formatting of the timeline view and the
datepicker labels when selecting annual datasets. The labels should show
only in `yyyy` instead of `MM yyyy` format when annual datasets are the
lowest common denominator selected.

### Notes & Questions About Changes
_{Add additonal notes and outstanding questions here related to changes
in this pull request}_

### Validation / Testing

- Add a dataset with annual time density and verify that the timeline
view labels and the datepicker labels show only the year
- Add datasets with month time density, and verify that now the labels
show the month and the year
- The rest should work as before (date picker shows the appropriate
calendar view)