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
