Instead of using image-uploader, which relies on the old
UploadMixin, we can now use the uppy-image-uploader which
uses the new UppyUploadMixin which is stable enough and
supports both regular XHR uploads and direct S3 uploads,
controlled by a site setting (default to XHR).
At some point it may make sense to rename uppy-image-uploader
back to image-uploader, once we have gone through plugins
etc. and given a bit of deprecation time period.
This commit also fixes `for_private_message`, `for_site_setting`,
and `pasted` flags not being sent via uppy uploads onto the
UploadCreator, both via regular XHR uploads and also through
external/multipart uploads.
The uploaders changed are:
* site setting images
* badge images
* category logo
* category background
* group flair
* profile background
* profile card background
