Disallow `Make a suggestion` button press if already have a suggestion in the comment

I often hit this situation

1. Start a PR comment
2. Press `Make a suggestion`
3. Type the suggestion
4. Get confused and think that the `Make a suggestion` button will submit the suggestion. Press it. Retract in shame.

We should disable this button once a suggestion is in the comment.

https://github.com/user-attachments/assets/94223707-45d7-48a1-a60a-8cc121d9259a

