closes https://github.com/TryGhost/Ghost/issues/8743
- the serializer in our mobiledoc renderer didn't have the list of non-closing HTML tags passed in which meant that tags such as `<br>` in the markdown HTML output were being re-serialized to `<br></br>` which is invalid HTML which (at least Chrome) then attempts to fix by rendering it as `<br><br>` instead
- the elements with incorrect rendering that may result in unwanted "fixing" by browsers are `AREA`, `BASE`, `BR`, `COL`, `COMMAND`, `EMBED`, `HR`, `IMG`, `INPUT`, `KEYGEN`, `LINK`, `META`, `PARAM`, `SOURCE`, `TRACK`, and `WBR`
refs #7429
- ☢️👷🏻♀️ This PR removes the dependency on Ghost-Editor and replaces it with the Mobiledoc DOM renderer. It includes new DOM based default cards and atoms.