Issue #3272985 by recrit, grasmash, catch, smustgrave, xjm: RSS Feed header reverts to text/html when cached

Issue #3085192 by Wim Leers, focus13, grasmash: Add index on source_ids_hash for migrate_message_* tables

Issue #2982684 by greg.1.anderson, Mile23, Mixologic, webflo, alexpott, yogeshmpawar, pingwin4eg, vijaycs85, larowlan, dww, borisson_, phenaproxima, kim.pepper, bojanz, grasmash, hctom, kmbremner, pingers, Jax, sherakama, derhasi, claudiu.cristea, jhedstrom, Xano, Grimreaper: Add a composer scaffolding plugin to core

Issue #2568413 by grasmash, pwolanin, dawehner, Wim Leers, xjm, klausi: REST views: Pass views style plugin instance to REST Export serializer