Prototype 2 series data model approaches and pick one

[cdrini]

Subtask of #6718

[cdrini]

Implemented Model Approach 1: Series as lists only, this seemed to work quite well, so moving forward with it for now.

The data model will be something like:

key: /lists/OL42L
type: { key: /type/list }
list_type: series
name: "Harry Potter"
description: "..."
seeds:
- { position: 1, item: { key: OL1W }, primary: true }
- { position: 2, item: { key: OL2W }, primary: true }
- { position: 0, item: { key: OL1513W }, primary: false }
- { position: 8, item: { key: OL1513W }, primary: false }
- { position: ‘1-7’, item: { key: OL12W }, compendium: true }

Note that this extends lists to have annotated seeds, as opposed to pure seeds.

Here are some screenshots from the demo:

A series page:

Editing a series page:

Creating a new global series:

Here is the proof of concept PR: #7804