Provide API to request description template names

The following discussion from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/32225#note_216260908 should be addressed:

• @stanhu started a discussion:

  Note that this is a private API that we are creating. It would be nice to expose this as a public API, too, but we can file a separate issue for that.

We could also consider providing this data via GraphQL.