Issue #2665992 by alexpott, klausi, heykarthikwithu, xjm, tstoeckler: @file is not required for classes, interfaces and traits

Issue #2625696 by chx, benjy, alexpott, catch, mikeryan, andypost, phenaproxima: Make migrations themselves plugins instead of config entities

Revert "Issue #2625696 by chx, alexpott, benjy, mikeryan, andypost, phenaproxima: Make migrations themselves plugins instead of config entities"

This reverts commit 694994fe.

This reverts commit f71ec898.

Issue #2625696 by chx, alexpott, benjy, mikeryan, andypost, phenaproxima: Make migrations themselves plugins instead of config entities

Issue #2429191 by claudiu.cristea, amateescu, yched, nlisgo, Berdir, alexpott, klausi, Wim Leers, xjm, catch: Deprecate entity_reference.module and move its functionality to core

Issue #2463909 by phenaproxima, mikeryan: Migrations should support non-installed default configurations (templates)

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