FEATURE: Move emoji uploader to use uppy (PR #14814)

This commit changes the emoji uploader to use the UppyUploadMixin, and makes some minor changes to the emoji uploader (tightening the copy for drag and drop and adding a percentage for the upload).

Since no other uppy upload mixin components have needed to upload multiple files so far, this necessitated adding a tracker for the in progress uploads so we know when to reset the uploader once all uploads are complete.

At the moment, the emoji uploader cannot be used for direct S3 uploads because the admin emoji controller creates other records and does other magic with the emojis. At some point we need to factor this kind of thing into the ExternalUploadManager.transform! action to complete external uploads.