Split metadata copying from mw.UploadWizardDetails
Created new uw.CopyMetadataWidget, which implements the interface and
logic for the "Copy information to all uploads following ..." feature.
Both were previously tacked onto mw.UploadWizardDetails.
The metadata copier is no longer part of mw.UploadWizardDetails, but
rather displayed alongside them. The interface changes are minimal now
but this should make it easier to change the design (T90741).
mw.UploadWizardDetails can now be serialized to a plain object,
same as uw.DetailsWidgets, and restored directly from that object.
This should allow for neat things like rescueing uploads in case of a
crash, or undoing changes made through "Copy metadata".