feat: support retrieving changelog from ghe hosted repositories (#1968)

lib/datasource/npm.js

lib/workers/pr/changelog/index.js

lib/workers/pr/changelog/source-github.js

lib/workers/pr/index.js

lib/workers/pr/release-notes.js

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

test/workers/pr/changelog.spec.js

test/workers/pr/index.spec.js

test/workers/pr/release-notes.spec.js