closes https://github.com/TryGhost/Ghost/issues/8958
- `markdown-it-named-headers` pulled in `String.js` despite it not being used which added a lot of String.prototype extensions and caused Ember deprecation notices
- moves the short `markdown-it-named-headers` functionality directly into our app code without using `String.js`
refs #718, refs https://github.com/TryGhost/Ghost/pull/8305
- meta description preview in the PSM was relying on the `html` field which is no longer queried - see #718 and https://github.com/TryGhost/Ghost/pull/8305
- restores live preview that was in LTS but removed whilst implementing mobiledoc because we had no quick way of rendering mobiledoc->text
- adds a boolean argument to the `formatMarkdown` util that can disable the replacement of `<script>` and `<iframe>` tags so that the inserted text isn't rendered when converting HTML to text
✨ use markdown-it for markdown previews
no issue
- replaces SimpleMDE's default `marked` rendering with `markdown-it`
- add ember-browserify and markdown-it plugins
