refs TryGhost/Ghost#8455
- ensure `uploadUrls` and `errors` are cleared in `gh-uploader` when new uploads are started
- yield `isUploading` in `gh-uploader` component
- replace image upload modals in settings/general with in-page uploads
no issue
* move "save on first change" behaviour into editor controller
* allow TAB events to be specified in keyEvents hash of gh-input
* replace mobiledoc-kit/gh-koenig with a SimpleMDE based editor
- remove `gh-koenig` in-repo-addon from `package.json` so that test files etc aren't loaded
- remove `mobiledoc-kit` dependencies
- extends `gh-editor` to handle file drag/drop
- adds `gh-uploader` and `gh-progress-bar` components to handle file uploads in a more composable manner
- adds `gh-simplemde` component that wraps SimpleMDE
no issue
- adds `gh-image-uploader` that handles image uploads in a fully ember fashion and with no dependency on `uploader.js`
- adds `gh-image-uploader-with-preview` that can fully replace the old `gh-uploader`
- replace uses of `gh-uploader` in PSM & TSM with `gh-image-uploader-with-preview`
- updates the editor preview image handling to use the new `gh-image-uploader-with-preview` component
- updates the image upload modal to use `gh-image-uploader` (utilises the `saveButton=false` flag which means the preview has to be handled externally to avoid auto-replacement when typing a URL)
- removes all old `uploader.js` related code
- adds custom `RequestEntityTooLargeError` and `UnsupportedMediaTypeError` errors to our `ajax` service
