Migration: post 1st iteration tasks

[zzacharo]

We should paste here all the tasks/feedback/comments generated after the 1st migration iteration.

Next tasks

### Migration tasks
- [ ] https://github.com/zenodo/zenodo-rdm/issues/337
- [ ] https://github.com/zenodo/zenodo-rdm/issues/344
- [ ] https://github.com/zenodo/zenodo-rdm/issues/345
- [ ] https://github.com/inveniosoftware/invenio-vocabularies/issues/183
- [ ] https://github.com/inveniosoftware/invenio-vocabularies/issues/251
- [ ] https://github.com/zenodo/zenodo-rdm/issues/314
- [ ] https://github.com/inveniosoftware/invenio-rdm-migrator/issues/85
- [ ] https://github.com/zenodo/zenodo-rdm/issues/311
- [ ] https://github.com/inveniosoftware/invenio-rdm-migrator/issues/86
- [ ] https://github.com/inveniosoftware/invenio-rdm-migrator/issues/87
- [ ] https://github.com/zenodo/zenodo-rdm/issues/341
- [ ] https://github.com/zenodo/zenodo-rdm/issues/342
- [ ] https://github.com/zenodo/zenodo-rdm/issues/343
- [ ] https://github.com/zenodo/zenodo-rdm/issues/284
- [ ] https://github.com/inveniosoftware/invenio-rdm-migrator/issues/88

### Migrator code improvements
- [ ] https://github.com/inveniosoftware/invenio-rdm-migrator/issues/6