RSS, photosets, figure_img S3 updates

- Fix for RSS files not including photos
- Added writing-only RSS feed
- New directory for RSS feeds
- Preparing for S3 upload workflow
- Images now live in a temporary S3 bucket
- Brand new, easier figure_img plugin syntax https://github.com/opattison/jekyll-figure-image-tag/commit/bccd4de75d4461a10d91810bdb28718aa21d485d
- Style changes for the excerpt-index