Issue #2066535 by tim.plunkett, jsst, Berdir: Add EntityManagerInterface and stop typehinting with EntityManager.

Issue #1953408 by swentel, aspilicious, andypost, yched: Remove ArrayAccess BC layer from field config entities.

Issue #2017851 by pcambra, amateescu, Pancho: Move entity_reference_get_selection_handler() to a method on SelectionPluginManager.

Issue #1950632 by effulgentsia, Wim Leers, amateescu, yched: Create a FieldDefinitionInterface, decoupled from  and  config entities, and use it for formatters and widgets.

Issue #1959116 by amateescu: Entity reference autocomplete shouldn't include html in its return value.

Issue #1801356 by dawehner, disasm, amateescu, effulgentsia: Entity reference autocomplete using routes.