Issue #1965208 by amateescu, Berdir: Convert TaxonomyTermReferenceItem to extend EntityReferenceItem.

Issue #1620010 by dawehner, ParisLiakos, plach, jibran: Move LANGUAGE constants to the Language class.

Issue #1818556 by das-peter, plach, fago, Berdir, vijaycs85: Change notice: Convert nodes to the new Entity Field API.

Issue #1253820 by yched, zserno, tim.plunkett, xjm, effulgentsia, plach: Fixed It's impossible to submit no value for a field that has a default value.

Issue #1778178 by fago, Berdir, effulgentsia, mradcliffe, das-peter: Convert comments to the new Entity Field API.

Issue #1552396 by andypost, sun, tim.plunkett, amateescu, YesCT: Convert vocabularies into configuration.

Issue #1711070 by kid_icarus, tim.plunkett, xjm, droplet, zendoodles and cam8001: Convert tests to use ::$modules property instead of parent::setUp($modules).

- Patch #1696902 by sun, larowlan, cosmicdreams, heyrocker, tobiasb: Convert forum_admin_settings() to new configuration system.