Date format and error in project template request

[eboileau]

A clear and concise description of what the bug is.

Date is optional, but as soon as we start typing something, we are forced to enter a full date: Please enter a valid date. This is fine. If we clear out one field, or any two fields (assuming only one or two fields were entered), this is still fine, but if we entered a full date, and clear out any field, then we get

date_published must be a `date` type, but the final value was: `Invalid Date` (cast from the value `""`). 

and we cannot get rid of this error, unless we enter a full date again.

We should also check if we can change the format to YYYY-MM-DD, which is used in the app. The date format in the form is mm/dd/yyyy.