This reverts commit 5dc2bd06.

Issue #2594565 by klausi, marthinal, dawehner, Berdir, yched: File extension + max_filesize should be validated using a Constraint on file fields

Issue #2607364 by plach, hchonov, catch: RC1 filled database update fails due to beta-12 to RC1 upgrade path issue

Issue #2598070 by Wim Leers, jaxxed, DuaelFr, Haza, Reinmar, oleq: [regression] CKEditor Link button does not show if HTML filtering is enabled

Issue #2585173 by Wim Leers, Jelle_S, DuaelFr, nod_, Reinmar, attiks: [regression] "Allowed HTML tags" setting corrupted upon accessing Text Format configuration UI

Issue #2510380 by Wim Leers, DuaelFr, mlewand, quicksketch, Reinmar, oleq: Images cannot be linked in CKEditor

Revert "Issue #2510380 by Wim Leers, DuaelFr, mlewand, quicksketch, Reinmar, oleq: Images cannot be linked in CKEditor"

This reverts commit 2d342c0f.

Issue #2510380 by Wim Leers, DuaelFr, mlewand, quicksketch, Reinmar, oleq: Images cannot be linked in CKEditor

Issue #2599190 by chegor, rakesh.gectcr, sdstyles, aneeshthankachan, anil280988, Himanshu5050, jhodgdon: Update upgrading procedure describing in UPGRADE.txt

Issue #2194155 by cosmicdreams, subhojit777, tatarbj, Cottser: Replace deprecated Twig_NodeInterface with recommended Twig_Node

Issue #2602872 by rakesh.gectcr: Wrong @param doc for construct function in class ContainerAwareEventDispatcher

Issue #2602464 by heykarthikwithu, anil280988, rakesh.gectcr: Remove unused local variable and add return value to docs in _search_find_match_with_simplify() of 'search.module'

Issue #2605388 by rakesh.gectcr, anil280988, mahavir003, aneeshthankachan: Comment Typos in DrupalKernel.php

Issue #2603372 by heykarthikwithu: Remove unused variable in LibraryDiscovery::getLibrariesByExtension()

Issue #2480333 by Wim Leers, Elijah Lynn: Document how to use an unminified CKEditor build (for developing/debugging)

Issue #2606460 by Wim Leers, alexpott: Regression caused by #2557113: Enabling CKEditor for a text format that did not have it previously is broken

Issue #2600672 by alexpott, dawehner: Core MarkupInterface implementations used in Twig templates should implement \Countable

Issue #2601790 by Liam Morland: In AttachedAssetsTest.php, separate a 3-part test into 3 separate test assertions

Issue #2597359 by Wim Leers: Require responses with attachments to contain the final attachment values

Issue #2324649 by phenaproxima, svendecabooter, benjy, Berdir: Migrate text fields correctly based on their text_processing setting

Issue #2596043 by mpdonadio, pjonckiere: DrupalDateTime constructor claims to support \DateTime object - it should not

Issue #2601682 by krknth, rakesh.gectcr, anil280988, jhodgdon: Unnecessary @param tags & Missing variable names in block, block_content module

Issue #2596083 by Wim Leers, lokapujya, jamin_melville: Allowed attribute 'class' not respected in CKEditor: ACF strips it

Issue #2549951 by Wim Leers: Document how to specify multiple CSS classes for CKEditor's styles dropdown

Issue #2604618 by andypost, xjm: Views operations dropbuttons do not work with Comment because it does not specify a list builder

Issue #2491875 by joachim, xjm, Xano, alexpott: EntityViewsData adds Operations links to all entities, which won't work if the entity type has no list builder, leading to WSOD on some views