Add discuss topic IDs. (#15)
Add discuss topic IDs. (#15)

This follows the [Discourse setup][] for embedding comments to
existing topics.

In a blog post, set the `discuss:` key the topic ID of the blog
post for the comments from https://discuss.dgraph.io.

If the blog post slug URL or the blog domain happens to change,
then this will maintain the same discussion thread for the post.
Otherwise, another discussion topic would be created in its place
when `discourseEmbedUrl` is different.

[Discourse setup]: https://meta.discourse.org/t/embedding-discourse-comments-via-javascript/31963
dgraph-io/hugo-dgraph-theme,dgraph-io/hugo-dgraph-theme