Cleaned up author data, removed from index/tag pages

This adds `authors` metadata to each blog post, with an array of team slugs (e.g. `robert` and `eric`) that match up to the values in `_data/team.yml`.

Where teammates don't appear in `team.yml` yet, I used their anticipated slug. Where we have guest authors, I've used their full name, in case that's useful.

In any case, it doesn't replace the byline - it looks like right now, we're using sentence-like bylines that would be difficult to generate automatically. Rather than try, I removed the display of `post.authors` from the index page and tag page, leaving the body display to show the byline (which typically appears at the top of blog posts).

In my next PR, I'll plan to make a custom tag for teammate names, that can be dropped in place of byline names, that automatically link the person's name to their authorial archive, and show a tiny version of their image next to them.