Update: change default role for custom elements

[scottaohara]

closes #2303

This updates a custom element's default role to none to allow attribute reflection from the custom element parent to its internals.

This change provides additional clarification about how a custom element can be provided a role by authors, and what caveats would change a custom element's default role of none, to another implicit minimum role.

Test, Documentation and Implementation tracking

Once this PR has been reviewed and has consensus from the working group, tests should be written and issues should be opened on browsers. Add N/A and check when not applicable.

• [ ] "author MUST" tests:
• [ ] "user agent MUST" tests:
• [ ] Browser implementations (link to issue or commit):
  • WebKit:
  • Gecko:
  • Blink:
• [ ] Does this need AT implementations?
• [ ] Related APG Issue/PR:
• [ ] MDN Issue/PR: