Let themes and modules provide content type templates

Problem/Motivation

Themes define the design that templates are built for, but each site has to write its templates by hand for each content type.

Steps to reproduce

Proposed resolution

• Themes and modules declare templates in their *.gutenberg.yml, each pointing to a JSON file:
  

  templates:
    article-hero:
      label: 'Article with hero'
      file: gutenberg/templates/article-hero.json
      template_lock: none
  

• The content type form offers them in a "Template source" select.
• A selected template is referenced, so the content type follows changes to it. "Customize" copies it into the content type's own template.
• Gutenberg Base ships an example.

Remaining tasks

User interface changes

The "Template source" select.

API changes

The templates key in *.gutenberg.yml.

Data model changes

The _template_source setting.