Issue #1784234 by linclark: Use schema.org types and properties in RDF mappings.

core/modules/forum/config/rdf.mapping.node.forum.yml

...modules/forum/config/rdf.mapping.taxonomy_term.forums.yml

core/modules/rdf/lib/Drupal/rdf/CommonDataConverter.php

core/modules/rdf/lib/Drupal/rdf/SchemaOrgDataConverter.php

...odules/rdf/lib/Drupal/rdf/Tests/CommentAttributesTest.php

core/modules/rdf/lib/Drupal/rdf/Tests/RdfaAttributesTest.php

.../modules/rdf/lib/Drupal/rdf/Tests/StandardProfileTest.php

...dules/rdf/lib/Drupal/rdf/Tests/TaxonomyAttributesTest.php

core/modules/rdf/lib/Drupal/rdf/Tests/UserAttributesTest.php

core/modules/rdf/rdf.module

core/modules/user/config/rdf.mapping.user.user.yml

...ndard/config/rdf.mapping.comment.comment_node_article.yml

...standard/config/rdf.mapping.comment.comment_node_page.yml

core/profiles/standard/config/rdf.mapping.node.article.yml

core/profiles/standard/config/rdf.mapping.node.page.yml

...ofiles/standard/config/rdf.mapping.taxonomy_term.tags.yml