Misc fixed to ELN with custom schemas. #826

examples/data/eln/example-schema.archive.yaml

gui/src/components/archive/ArchiveBrowser.js

gui/src/components/archive/metainfo.js

gui/src/components/uploads/CreateEntry.js

gui/src/components/uploads/UploadPage.js

nomad/datamodel/context.py

nomad/datamodel/data.py

nomad/datamodel/metainfo/eln/__init__.py

nomad/metainfo/metainfo.py

tests/datamodel/test_context.py