Comments that use only inline directives should default to an "Informing" disposition

It'd be nice if comments (maybe only/especially top-level comments) whose body consists only of [inline directives](https://docs.reviewable.io/#/discussions?id=inline-directives) defaulted to the "Informing" disposition.

Otherwise, it's easy for people to accidentally block a PR when all they really want to do was add a reviewer/label, and makes the inline directives more tedious to use correctly.