Series feature refactor, redux

[lyzadanger]

Third time is the charm?

I realized that:

• series is content, not data; there should be one content module (template) per
  • Use a tag for this
  • Paginate the series collection
• content (posts) can use series instead of inSeries key in front matter to associate with series
• No need to create custom collections, one per series; just filter collections.all on the series slug with a custom filter or similar