refs https://github.com/TryGhost/Ghost/issues/9623
- added `DomModifier` class to walk a SimpleDom document and modify as needed
- adds `id` attributes to `h1`, `h2`, etc heading tags
- converts H* tag content to a dasherized string for the id attribute (dasherized id's are different to the smushed ids that are generated by our markdown converted but there are no backwards-compatibility concerns here)
- if a duplicate id is detected then add a `-1`, `-2`, etc suffix to the id
- use `DomModifier` after converting mobiledoc to SimpleDom but before serialising to html
- switched top-level var declarations to es6
refs https://github.com/TryGhost/Ghost/issues/9505
- updates mobiledoc converter's `render` method to accept a `version` argument
- `1` === Ghost 1.0's markdown-only renderer output
- `2` === Koenig's full mobiledoc renderer output
- switch between mobiledoc renderer versions in Post model's `onSaving` hook
- version 1 by default
- version 2 if Koenig is enabled (currently behind dev experiments config + labs flag)
- version 2 if the post's mobiledoc is not compatible with the markdown-only renderer
- "version 2" full-Koenig mobiledoc renderer output
- wraps content in a `.kg-post` div
- removes wrapper around markdown and html card output
- adds classes to image card output including selected image size/style
- standardises es6 usage across mobiledoc related files
