Add comment preview button
Summary
Currently it is not possible to view how your comment will look before publishing.
Description
When adding comments in aur, user may be unsure about the markup (how it will be rendered). There only way to see final result is to publish your comment. While it may be unwanted to publish yet, because the markup may be wrong, but all the subscribers will receive notification with that yet unready comment.
It would be good to add a Preview button, similar to what you can see in the Gitlab comments.