Issue #2845483 by heddn, holingpoon, Jo Fitzgerald, quietone, phenaproxima, claudiu.cristea: Rename Iterator process plugin and add documentation

Issue #2618332 by phenaproxima, jonhattan, rakesh.gectcr, mikeryan, neclimdul: Better handle replacement of missing filters with filter_null

Revert "Issue #2560795 by mikeryan, phenaproxima, penyaskito, willwh, benjy: Source plugins have a hidden dependency on migrate_drupal"

This reverts commit 89815ba3.

Issue #2560795 by mikeryan, phenaproxima, penyaskito, willwh, benjy: Source plugins have a hidden dependency on migrate_drupal

Issue #2549077 by pwolanin, Wim Leers, phenaproxima, lauriii: Allow the "Limit allowed HTML tags" filter to also restrict HTML attributes, and only allow a small whitelist of attributes by default

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

Issue #2460529 by alexpott, chx, phenaproxima: Migrations need to use the configuration entity dependency system

Issue #2466797 by mikeryan, benjy: Rename migration_groups key to avoid confusion with MigrationGroup support

Issue #2121299 by andypost, bdone, benjy, penyaskito, chx, claudiu.cristea, damiankloip, gregboggs, InternetDevels, jessehs, jhedstrom, marvil07, mikeryan, pcambra, Xano, YesCT, axoplasm, kgoel, dclavain, jeckman, alvar0hurtad0, Drupali, fastangel, gloob, jhedstrom, joshtaylor, mpgeek, alexpott: Migrate Drupal 6 to Drupal 8

Issue #2233787 by alexpott, xjm | tim.plunkett: Move default configuration into extension/config/install to clarify its purpose.

Issue #2213451 by andypost, bdone, benjy, penyaskito, chx, claudiu.cristea, damiankloip, gregboggs, InternetDevels, jessehs, jhedstrom, marvil07, mikeryan, pcambra, Xano, YesCT: Update Migrate API in core