Add missing MJML components

[caseyjhol]

The following MJML components are currently missing:

• [x] mjml-accordion
• [x] mjml-carousel
• [x] mjml-head-breakpoint
• [x] mjml-head-html-attributes
• [x] mjml-hero
• [x] mjml-navbar
  • [x] mjml-navbar-link
• [x] mjml-social
• [x] mjml-spacer
• [x] mjml-wrapper

I'll begin work on a PR and check off components as I work my way through the above list.

[FelixSchwarz]

All missing components were implemented as part of #38 so I think we can close this issue!