accessibility issues

**Issue 1**

When the user uses keyboard to navigate in the date picker modal with year/month dropdown enabled, the user is not able to set a date successfully using the year/month dropdown.

Steps to reproduce:
- open the date picker modal.
- use arrows to set the correct day, and hit tab to navigate to the year dropdown menu
- use arrows to set the correct year, and then hit tab to navigate to the month dropdown menu
- use arrows to set the correct month
- hit enter key, but the picker will not close, and the set date will not show in the input field
- hit tab key, the focus moves to "Today", then "Clear", then "Close", and then the picker is closed, but the set date does not show in the input field

Expected behavior:
After the correct month is selected from the month dropdown menu, the user should be able to set the selected date to the input field and close the modal

Suggested solution:
Provide a "Done" button

**Issue 2**

The user cannot use keyboard to trigger prev and next icons.

Steps to reproduce:
- Open date picker modal
- Navigate within the modal using tab
- The prev and next icons will not receive focus, and hence cannot be used with keyboard

Expected behavior:
The user should be able to use prev and next icon with keyboard

Suggested solution:
Add tabindex to prev and next icon

**Issue 3**

Users who tend to set date in a year > month > day logic cannot navigate back to select the day after selecting month from dropdown menu.

Steps to reproduce:
- Open date picker modal
- Use tab key to navigate to month dropdown, and then hit tab key again

Expected behavior:
- Users should be able to navigate back to the day area to set or change the day

Suggested solution:
Close the date picker modal only when the user hit enter on "Close" button, or hit "ESC". Do not close the modal after the user navigates to "Close" button.