Niche targeting explainer page

[davidfischer]

• Add a page explaining niche targeting and link from the advertiser page
• Cross-link with the similar pages demo
• Fixed a minor bug with prefilling checkboxes
• Should we link to our blog posts on the subject from the FAQ?

[ericholscher]

Feel free to merge this when you think it's ready. No need for me to take a second look. 👍