Validate markdown upon comment/thread creation

[themmes]

“Something went wrong” messages showing on the forum after submission without any understandable feedback.

[twahtera]

It seems that containing the underscore character "_" is enough to trigger this.

[twahtera]

I initially thought that would have been a mmark bug but seems that it's documented behaviour and mmark just handles underscores differently from CommonMark and many other markdown implementations so yes, just validating the message before submit would work.