Allow to edit a comment

[yammesicka]

We should NOT change the CommentText itself.
This is a mutable behavior we prefer to avoid whenever we can, and may mistakenly change previous comments of the author.

Instead, we should check if the new text is already in CommentText.
If it is exist in CommentText, we update Comment so it would refer to the CommentText foreign key.
If not, we should create a new CommentText.

[Gonzom]

I'll take this one 👍