facets: provide CombinedTermsFacet to fix #798 [+]

[fenekku]

This approach doesn't use 'nested' fields, but instead relies on a field containing <parent><split char><child> to aggregate correctly.

• Alternative to: https://github.com/inveniosoftware/invenio-records-resources/pull/546
• Part of closing: https://github.com/inveniosoftware/invenio-rdm-records/issues/798

[fenekku]

the mergening has started