feat(pr): simpler displayFrom/displayTo logic (#3853)

lib/config/definitions.js

lib/workers/pr/pr-body.js

lib/workers/repository/updates/generate.js

renovate-schema.json

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

test/workers/pr/index.spec.js

...ers/repository/updates/__snapshots__/flatten.spec.js.snap

...rs/repository/updates/__snapshots__/generate.spec.js.snap

test/workers/repository/updates/generate.spec.js