Use shadow DOM to prevent style leaks

[ifvictr]

Goal

Stop webpage styles from leaking into doorhanger

Todos:

• Add shadow DOM logic
• Get Linaria styles to work
• Fix regression of doorhanger open transition

Implementation Decisions

All Submissions:

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?