Follow up to ac672cf. Fixes a
small issue with uppy-image-uploader where the Processing label
was shown for the whole upload. Also adds a couple of options to
pick-files-button to allow for it to be used in the uppy-image-uploader.
Also fixes an issue where the uppy-upload mixin was resetting prematurely
when all uploads in progress were complete, but it should have been doing
that on the uppy complete event instead.