Redesign the homepage episode list

[dannycolin]

This patch groups each episode year in a <details> that can be expanded/collapsed. On browsers supporting name= on <details>, only one group is expanded and the other ones are auto-collapsed on group change.

The /assets directory is a special directory that jekyll monitors. Since I added an app.css file, I had to move all files we don't want jekyll to monitor in a /miscs.

You might have to update your dev environment:

# Update dependencies
bundle update

# Update bundler itself
bundle update --bundler