UploadWizard: Use prefix for class names for css-scoping

"Busy" is probably a class that is not too uncomon. To avoid side effects,
prefix it like all class names in this extension with `mwe-upwiz`.

Change-Id: Ia37d5bc3fdbdc27b332f883cfd9ccb676132c652
Merge "UploadWizard: Use prefix for class names for css-scoping"