Update DAM template for 5.8

The "DAMThumbnailLayout" XML extension has a bug: the name of the widget was changed (maybe in 5.7.3, eventually 5.7.2?). So changing the "DAMThumbnailLayout" form layout has no impact once the project is deployed on the server.

So, in the template, the "DAMThumbnailLayout" extenstion must be changed, using "dam_listing_metadata" instead of "dam_listing_dam_common" for the name of the widget:

<extension target="org.nuxeo.ecm.platform.forms.layout.WebLayoutManager" point="widgets">
    <widget name="dam_listing_metadata" type="layout">
    . . .