Allow Birthday field to exclude a Year

[jwflory]

Tested on 4.0.0-beta.11.

Summary

In the Birthday metadata field, allow input of a month and a day without specifying a year.

Background

For many of my contacts, I only know their birth month and birth day. I do not know their birth year. This is behavior I find in other contact apps like Google Contacts. As a temporary workaround, I specify 1901 where I do not know the contact's birth year. But instead of false data, I'd prefer to omit.

Details

Allow a Birthday field to only a month and day. Add validation if user does not specify a year, but make sure it is a valid input for month and day.

Outcome

Easier to store useful information about my contacts without creating false data