community: a11y improvements for community modals and items

[jennur]

Closes https://github.com/inveniosoftware/invenio-app-rdm/issues/2182

NEEDS https://github.com/inveniosoftware/react-invenio-forms/pull/199

• Fixes accessibility for "Submit to community" modal and CommunityItem
• Some layout changes, aligned with https://github.com/inveniosoftware/invenio-app-rdm/pull/2185 <<-- see screenshots

[jennur]

This should be moved to https://github.com/inveniosoftware/invenio-rdm-records/tree/master/invenio_rdm_records/assets/semantic-ui/js/invenio_rdm_records/src/deposit/components and/or https://github.com/inveniosoftware/invenio-communities/tree/master/invenio_communities/assets/semantic-ui/js/invenio_communities/community/communitiesItems