Revert "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 079a52b4.

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 #2458925 by alexpott: Screen is black and completely unreadable in Configure page after install on standard profile

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

Issue #2411689 by alexpott: Use a MemoryBackend in StorageComparer so that configuration import validators don't have to reread data from disk or the db

Issue #2454859 by Gábor Hojtsy, alexpott, mrjmd: Not possible to format plural an already translated string

Issue #2458993 by Wim Leers: #cache[expire] is undocumented, unused, untested: remove it, use #cache[max-age] instead

Issue #2458413 by Wim Leers: BlockViewBuilder should specify cache contexts even for uncacheable blocks

Issue #2456707 by dawehner, larowlan: Block Content views field handlers need to be replaced with entity-aware handlers

Issue #2428695 by amateescu, daffie: SQLite date handling is wrongly implemented and arguments handling needs override

Issue #2458289 by Wim Leers: CronRunTest::testAutomaticCron() should test with an authenticated user

Issue #2450637 by tim.plunkett, jasonawant: Block page visibility option "Show for the listed pages" does not appear to save

Issue #2372011 by richardcanoe, Noe_: NotFoundHttpException handler should take into account it's parameters

Issue #2457251 by vijaycs85: Remove unnecessary call to drupal_get_user_timezone() in Drupal/Core/Datetime/DrupalDateTime::prepareTimezone() method

Issue #2408511 by joaogarin, pjbaert, rpayanm, bobrov1989: Rewrite vertical-tabs component inline with our CSS standards

Issue #2454441 by rteijeiro, hussainweb, 17thColossus, nullkernel: Rename Typed Data classes to support PHP 7

Issue #2432837 by Wim Leers, Fabianx: Make cache contexts hierarchical (e.g. 'user' is more specific than 'user.roles')

Issue #2454447 by andypost, dawehner, rteijeiro: Split Utility\String class to support PHP 7 (String is a reserved word)

Issue #2090115 by alexpott, xjm: Don't install a module when its default configuration has unmet dependencies

Issue #2394419 by hussainweb, cilefen, sachinwable: Clean-up file module test members - ensure property definition and use of camelCase naming convention

Issue #2373741 by Wim Leers, hussainweb: Move bulk of render pipeline documentation to d.o handbooks

Issue #2448357 by amateescu: Config entity forms need to have access to an updated entity object at all times

Issue #2303765 by davidhernandez, larowlan, mgifford: Make the default 'alt' attribute for Image fields required

Issue #2455415 by rteijeiro, hussainweb, rpayanm, baisong: Rename Views plugin classes to support PHP 7

Issue #2444211 by Wim Leers, jhodgdon: Document cacheability of render arrays, and the considerations to use when generating render arrays

Issue #2388349 by almaudoh: FormState::cleanValues() doesn't conform with the FormStateInterface documentation

Issue #2456459 by andypost: CommentInterface::getCommentedEntity() should return FieldableEntityInterface

Issue #2148319 by crazyrohila, juankvillegas, Manjit.Singh, poojakural, Chloe Chen, joshtaylor: h2 on footer blocks must be hidden by default but must show when its enable