Content should be able to be nested under other content

Expected behavior

• Instead of just a one-dimensional list of content under each content type (characters, locations, etc), we should allow each piece of content to have a parent and/or child content.
• We also need to figure out how to rearrange things in the UI to make these relationships clear to set, while also avoiding people "losing" nested content (that might not otherwise be in e.g. the top-level Characters list).

Implementation steps

• Add parent_content_id and parent_content_type fields on all content types to add a polymorphic hierarchy of content linking across all types
• Add a child_content method to content (probably through a concern so we can reuse it across all types) for getting all children content

Design

• Rework the sidebar from a list of content types to a hierarchical view of a user's content, with content-without-a-parent nested under the (current) content types. For example:

Characters
  Bob
  Alice
Locations
  Empiria
    Citytown
      Mr. Mayor
      Superchurchplace
    Towntown
Magic
  Elemental Magic
    Fireball
  Nature Magic
    Earthball

• Since we can nest any kind of content under any other kind of content, we want to include an icon for the content type next to each name. So, for example, Mr. Mayor should have a character icon next to it in the above list.
• When a user only has one universe, omit the Universes section in the sidebar.
• When a user has multiple universes, use them as the top-level elements in the hierarchy and nest Characters, Locations, Items, etc under the selected universe.
• All content should be collapsible/expandable.

Here's a screenshot of a user who offered this as feedback on how they're organizing their magic structure:

No files, tests, or entry points are named. Start by reviewing the implementation steps and sidebar design checklist in this issue; done means content can be nested across types and the sidebar clearly represents, expands, and collapses those relationships without hiding content.