Automatically add merge requests merged into stable branches to the blog post.

As part of the maintenance policy extension &828 (closed), engineers will be able to merge bug fixes into stable branches. When a patch release is prepared, release-tooling will consider the merge requests already merged into stable branches to prepare the blog post. If one merge request is merged after the blog post was built, there is no automated way for this one to be added to the blog post, instead release managers have to manually added.

Given patch releases will be prepared based on demand or on the patch release pressure, adding entries to the blog post once created might be an unusual case, still it'd be nice to have an automated way of doing so, for example, a command could be added to update the blog post, e.g /chatops run blog_post update, this command will:

1. Fetch the current blog post
2. Review if the merge requests merged into stable branches are mentioned in the blog post
3. If not, it will add the missing entries.