custom fields: use `VocabularyRelationSchema`

[ppanero]

• related to https://github.com/inveniosoftware/invenio-vocabularies/issues/188

• [x] implement BaseVocabularyRelationSchema. With id, so others like generic and contrib can extend.

• [x] implement VocabularyRelationSchema and others needed for contribs, inheriting from the first one.

• [x] Remove the usage of VocabularySchema in invenio-communities and invenio-rdm-communities

• [x] Use VocabularyRelationsSchema as returned field in the VocabularyCF.field function. Need to use fields.Nested.

might need also UNKONW=EXCLUDE

[ppanero]

PRs were merged