This is a big change to change over to using the uppy
upload mixin in the composer by default. This gets rid
of the temporary composer-editor-uppy component, as well
as removing the old ComposerUpload mixin and copying over
any missing functions that were not yet implemented by
ComposerUploadUppy. This has been working well on our
hosting for some time now and has led us to several
bug fixes.
This commit also deletes the old plugin API for adding
preprocessors for the uploads. The accepted method of doing
this now is via an uppy preprocessor plugin, which we have
several examples of in the core codebase.
Leaving the `enable_experimental_composer_uploader` site setting
intact for now because some plugins still rely on it, this
will be removed at a later date.
One step closer to ending the jQuery file uploader saga...
