Update CMS to handle all person records and add fellow showcase category

[mradamcox]

This PR includes

• a handful of style updates across the site, especially focused on the news posts content
• improved preview style in the CMS that matches the published style of blog posts
• migrates all person records from the single JSON file we had before into the CMS, so they can be easily edited by anyone
• adds a new category to the CMS "Showcase" that will hold entries for fellow project showcases
• adds a new template and layout for showcase entries
  • all entries are now listed at /showcase
  • individual writeups are located at /showcase/{slug}

Closes #3 (by adding a hint to the field) Closes #21 (better instructions in readme for captions, and styles make them work in preview) Closes #171 (adds new CMS category and template) Closes #229 (also making links more consistent across the site)

I also did some general style fixups, like making text in all pages the same size and better line-height spacing especially in blog posts.

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