Reinstate "CubeList._repr_html_" method.

[pp-mo]

🐛 Bug Report

How To Reproduce

CubeList([])._repr_html_() ought to produce an html string, but instead fails with a simple AttributeError: 'CubeList' object has no attribute '_repr_html_'

Notes

Seems to be a bug introduced in #4767

We already test the behaviour of CubeListRepresentation.repr_html() But tests don't call CubeList._repr_html_ itself.

Fix should include a test that CubeList._repr_html_

1. exists,
2. creates a CubeListRepresentation, and
3. calls its repr_html (can be mock-ist)