[candidate_parameters] Add date precision field for DoD

[CamilleBeau]

Brief summary of changes

This PR adds a field to indicate the precision of DoD known as discussed in the LORIS meeting.

The options for precision are "Known year and month", "Known year" and "Unknown Date". If "Unknown date" is selected, the date field should not be entered.

Testing instructions (if applicable)

1. Go to candidate parameters for a participant to update the DoD
2. Make sure you can not save without a precision selected
3. If "Unknown date" is selected, make sure you can not fill out a date / the date is not required.

Link(s) to related issue(s)

• Resolves # (Reference the issue this fixes, if any.)

[nicolasbrossard]

Check notes from LORIS meeting for more info.