Fix metadata layout height on custom documents

Steps to reproduce:

• Create custom doctype that extends File
• Add a String and a Date field to the schema
• Go to Designer and click configure on Create, Edit, View and Metadata layouts to instantiate them (but don't change anything)
• Go to Nuxeo, hot reload, and create one of your documents
  => The metadata view has all the fields, but is a tiny little strip that has to be scrolled to see anything. (see attached screenshot)

Workaround:

• Add nuxeo-document-viewer element to View layout.

Expected behaviour:

• Metadata layout height should not be affected by View Layout's content or lack thereof.
  AND/OR
• The default configuration when instantiating a View layout should include nuxeo-document-viewer.