Issue #1392754 by tss, justAChris, jhodgdon: Comply with new documentation standards for @file for namespaced class files

Issue #2444231 by Wim Leers, nlisgo: Fix CacheableInterface so it makes sense (was: "Make Config objects & Entities implement CacheableInterface + add BubbleableMetadata::createFromCacheableObject()")

Issue #2428703 by Wim Leers: Add a 'user.permissions' cache context (was: "Should cache contexts be able to associate a cache tag?")

Issue #2271251 by ParisLiakos, undertext, JeroenT, StevenPatz, tim.plunkett: Remove usages of watchdog() from non-form and non-controllers.