fix: Schedule should always be an array (#580)

docs/configuration.md

docs/faq.md

lib/config/definitions.js

lib/config/migration.js

lib/config/validation.js

lib/workers/branch/schedule.js

test/config/__snapshots__/index.spec.js.snap

test/config/__snapshots__/migration.spec.js.snap

test/config/index.spec.js

test/config/migration.spec.js

test/config/validation.spec.js

test/workers/branch/schedule.spec.js