Create a schema for "allowed_html" which provides a better config diff

Problem/Motivation

I am working on a site which requires quite a specific and detailed element/attribute whitelist. My "allowed_html" setting is 1200 characters long. When updating this, it's impossible to tell from a diff what changed. In addition to this, we've found that changes manually made to the attributes and classes are reverted, chopped and changed by Drupal.behaviors.filterFilterHtmlUpdating erroneously, possibly from a bug in the JS or one of the filters we have installed.

It would be great if the schema that stored these settings provided a better diff.

Proposed resolution

Update the schema to look something like:

    allowed_html:
      type: sequence
      label: 'Allowed HTML'
      sequence:
        type: sequence
        label: 'Attributes'
        sequence:
          type: string
          label: 'Value'

Which would transform settings like:

allowed_html: '<a href hreflang> <em> <strong> <cite> <blockquote cite> <code> <ul type> <ol start type>'

Into:

      allowed_html:
        a:
          href: '*'
          hreflang: '*'
        em: {  }
        strong: {  }
        cite: {  }
        blockquote:
          cite: '*'
        code: {  }
        ul:
          type: '*'
        ol:
          start: '*'
          type: '*'

Remaining tasks

Validate & see if this is something we can achieve without breaking BC.

User interface changes

API changes

Data model changes