Issue #2711099 by heddn, Jo Fitzgerald, edysmp, edgewl2, alexpott, Charlotte17, mikeryan, masipila, quietone, maxocub, phenaproxima: Categorize migrations according to their type

Issue #2917883 by quietone, TR, Jo Fitzgerald, masipila, phenaproxima: Rename 'migration_templates' directories in core modules to 'migrations'

Issue #2620364 by quietone, bburg, Jo Fitzgerald, Yogesh Pawar, amoebanath, phenaproxima, Gábor Hojtsy, catch: Variable to config: theme settings [d7]