Revert "Issue #2457653 by Gábor Hojtsy: System.site langcode is both used as a file language code and a site language code"

This reverts commit 364a6e9c.

Issue #2457653 by Gábor Hojtsy: System.site langcode is both used as a file language code and a site language code

Issue #2014955 by YesCT, penyaskito, kfritsche, balintcsaba: Deleted bundles do not have their language configuration deleted

Issue #2408439 by zuuperman, askibinski: Language / Translation settings are not saved when adding a new bundle

Issue #2395395 by alexpott, effulgentsia, Gábor Hojtsy: TestBase lacks a config method to be used consistently in tests

Issue #2355909 by penyaskito, alexpott, Gábor Hojtsy, DuaelFr: language.settings config is not scalable

Issue #2340667 by YesCT, Pedro Lozano, alexpott, martin107: Protect Drupal\Core\Language\Language::id, and use getId().

Issue #2337825 by YesCT, martin107, mon_franco: Update comment references to the interface for ConfigurableLanguage class due to the ConfigurableLanguageInterface (followup).

Issue #2339435 by YesCT, alexpott: Default no longer needs to be a property on Language or ConfigurableLanguage.

Issue #2336177 by alexpott, YesCT: Fixed ConfigurableLanguage cannot be created/removed directly, integrate language_save() and language_delete().

Issue #2271005 by martin107, YesCT, alimac, mrjmd: Rename Language module's LanguageInterface to ConfigurableLanguageInterface and Language to ConfigurableLanguage.

Issue #697760 by sun: Replace getInfo() in tests with native phpDoc + annotations (following PHPUnit).

Issue #2166915 by ianthomas_uk, sun | plach: Remove uses of deprecated language functions in tests and procedural code.

Issue #2070369 by herom, bdgreen, Mac_Weber, InternetDevels, sidharthap, JeroenT: Remove all calls to deprecated function language().

Issue #2074037 by Wim Leers, effulgentsia: Add drupalPostUrl() — drupalPost()/drupalPostAjax() are for forms only, D8 JS performs non-form HTTP requests.

Issue #1754246 by webflo, swentel, penyaskito, andypost, alexpott, YesCT, dagmar: Languages should be configuration entities.

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

Issue #1739928 by vasi1186, YesCT, webflo, Transition, plach, Gábor Hojtsy: Generalize language configuration on content types to apply to terms and other entities.

Revert "Issue #1588422 by andypost, disasm, sun, dawehner, vasi1186, alexpott, tim.plunkett, larowlan, Gábor Hojtsy: Convert contact categories to configuration system."

This reverts commit 5cb63c4d.

Oops. That was a little more patch than we bargained for. :P

Issue #1588422 by andypost, disasm, sun, dawehner, vasi1186, alexpott, tim.plunkett, larowlan, Gábor Hojtsy: Convert contact categories to configuration system.