feat: customisable prFooter (#1648)

lib/config/definitions.js

lib/config/templates/default/pr-body.hbs

lib/config/templates/docker-digest/pr-body.hbs

lib/config/templates/docker-group/pr-body.hbs

lib/config/templates/docker-pin-group/pr-body.hbs

lib/config/templates/docker-pin/pr-body.hbs

lib/config/templates/docker/pr-body.hbs

lib/config/templates/group/pr-body.hbs

lib/config/templates/lock-file-maintenance/pr-body.hbs

lib/config/templates/node/pr-body.hbs

lib/workers/global/index.js

lib/workers/pr/index.js

lib/workers/repository/onboarding/pr/index.js

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

test/manager/__snapshots__/resolve.spec.js.snap

test/workers/pr/__snapshots__/index.spec.js.snap

test/workers/pr/index.spec.js