Issue #1026616 by fgm, Berdir, Dave Reid, fago: Implement an entity render controller.

core/includes/entity.api.php

core/includes/entity.inc

core/lib/Drupal/Core/Entity/EntityRenderController.php

...ib/Drupal/Core/Entity/EntityRenderControllerInterface.php

core/modules/comment/comment.module

core/modules/comment/comment.pages.inc

core/modules/comment/lib/Drupal/comment/Comment.php

...es/comment/lib/Drupal/comment/CommentRenderController.php

...nt/lib/Drupal/comment/Tests/CommentContentRebuildTest.php

core/modules/field/field.multilingual.inc

core/modules/file/file.module

core/modules/node/lib/Drupal/node/NodeRenderController.php

...dules/node/lib/Drupal/node/Tests/NodeBuildContentTest.php

core/modules/node/node.module

.../lib/Drupal/search/Tests/SearchMultilingualEntityTest.php

core/modules/search/search.api.php

...les/taxonomy/lib/Drupal/taxonomy/TermRenderController.php

core/modules/taxonomy/taxonomy.module

core/modules/taxonomy/taxonomy.pages.inc

core/modules/user/lib/Drupal/user/UserRenderController.php