closes https://github.com/TryGhost/Ghost/issues/7144
- allow the `accept` attr of `gh-file-uploader` and `gh-image-uploader` to be specified
- allows a `validate` action to be passed into `gh-image-uploader` and `gh-file-uploader` components that runs after a file is selected and before the upload starts
- adds a default `validate` action to `gh-image-uploader` and `gh-file-uploader` that triggers the normal `UnsupportedFileType` error when the selected file's mime-type does not match the `accept` attribute
- adds mime type validation to labs importer (basic implementation, should be replaced with uploader components once they have been refactored)
no issue
- override `x-file-input` in `gh-file-input` to look for a custom property on the change event if we are in testing mode (this is necessary because Ember 2.5+ use native rather than jQuery events so `target.files` is readonly, see https://github.com/emberjs/ember.js/issues/13540)
- migrate unit tests for the uploader components to the integration tests
- add skipped acceptance tests for the subscribers CSV import now that it's possible to simulate file uploads
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
