Editor & Submission: Use a dropdown for country input

[StevenDufresne]

We currently use the country meta value. It's probably a better approach to use a custom taxonomy to help with data input.

[ryelle]

@StevenDufresne Do you still think this is worth adding, even though we probably rarely have this data?

[StevenDufresne]

Probably not, the main issue was that it relie(d/s) on custom fields which isn't a great input interface.

[ryelle]

Ah, I see, it was about inputting data, not for front-end filtering.

The submission form uses a dropdown, so maybe we should switch the meta field to also use a dropdown. I don't think we need to change the value storage right now, I think there's a helper function that can get a list of countries we've used on WordCamp.org, so we don't need to create that list from scratch.

Updated the title to reflect the issue.