Add question about having Contributor Day to WordCamp application

[timiwahalahti]

Since WordCamps might have Contributor Day and we have fields for that on the WordCamp tracker, we should ask about that also during the application process. Right now the Deputy vetting the application or mentoring WordCamp needs to ask this in order to get the tracker data to reflect the right situation.

This PR adds a question about organising Contributor Day and options to choose at which point of the WordCamp they are planning to have it.

Fixes #276

Screenshots

Screenshot before the field was added.
Screenshot after the field was added.

How to test the changes in this Pull Request:

1. Go to the WordCamp Organiser application page
2. Fill out and submit the application, new field should be visible
3. Check the application on WordCamp tracker, Contributor Day checkbox is checked and original application rows contain the answer

[timiwahalahti]

Tested this old PR myself and made sure it's still something that is needed. Ready for the review!

[pkevan]

@hellojuliarose is this something we want to add on the WordCamp and/or Events application form?

[outdoor2kode]

Checked the code and did testing. It looks good.

However, before merging, I'd like to follow up on the question Paul previously raised and ask @WordPress/community-admins if this feature is needed.

Would this help in vetting WordCamp applications?

After this PR is shipped, the application form will include an additional question asking if there is a contributor day. If the applicant answers yes, the contributor day checkbox will automatically be checked off in the WordCamp tracker, and you can also find out when the applicant plans to hold it through the "original application" section at the bottom.

[dorsvenabili]

However, before merging, I'd like to follow up on the question Paul previously raised and ask @WordPress/community-admins if this feature is needed.
Would this help in vetting WordCamp applications?

Yes. I think this extra info in the application form would help us with the vetting process and it'd be useful for our stats.

Thank you :)