Add first time participation measurement

[renintw]

See #888

This PR introduces a measurement for first-time participation of organizers, sponsors, speakers, and volunteers into the WordCamp Counts table. It not only presents the total number of first-time participants but breaks down these figures for each individual event within a specified time period.

Screenshots

How to test the changes in this Pull Request:

1. Sandboxed.
2. WP Admin Dashboard => (Top-left side) Dashboard => Reports => WordCamp Counts
3. Select the desired date range and then click on Show results.

[iandunn]

This is looking good 👍🏻

[devinmaeztri]

Thank you @renintw and @iandunn! I've tested, and it looks good to me. At first, I was confused with FT and G and the x/x/x. But I got it now. Just to confirm, is the ? for folks who opt out, and who don't fall into either option?

[renintw]

Hey @devinmaeztri 🙂

In the case of Gender, ? would be folks whose gender is unknown. Currently, gender stats here aren't the ones filled in by users. It's inferred from users' names through genderize.io. And since the gender is identified via the system, the sum of F+M+? would always equal the Total. (The system will always categorize an individual's gender into one of these three options)

In terms of FT, ? is unsure, indicating folks who are unsure about whether they are the first time being a role. This value is filled in by users themselves when applying for a certain role.
For example, when a person applies for being the speaker, there's a question in the application form asking if they are first time being a speaker. If they select I don't know, then it would be a ? in the report here (or Unsure in the Speaker post edit page ⬇️ ).

And since the said question is optional, some users may choose to opt out. This opt-out decision would not be categorized into any of the options (Y/N/?), so the sum of Y+N+? here doesn't always equal the Total.