Add description_html fields to labels API responses
For gitlab-ui#466 (closed), @fguibert needs our REST API to expose a label's description_html
field in a few places:
- For individual labels: https://docs.gitlab.com/ee/api/labels.html
- For any 'labelable' resource (https://docs.gitlab.com/ee/api/issues.html, https://docs.gitlab.com/ee/api/epics.html, https://docs.gitlab.com/ee/api/merge_requests.html).
This should be cheap as it's stored in the DB. If it's not going to cheap, please let me know and we can consider a change.