refactor: Move generateConfig code from branchWorker to repositoryWorker (#449)

lib/workers/branch/index.js

lib/workers/repository/index.js

lib/workers/repository/onboarding.js

lib/workers/repository/upgrades.js

test/workers/branch/index.spec.js

test/workers/repository/__snapshots__/upgrades.spec.js.snap

test/workers/repository/index.spec.js

test/workers/repository/onboarding.spec.js

test/workers/repository/upgrades.spec.js