Add busy state to Add comment field after adding a comment

Description

When I add a comment, this happens

Only the button changes its state. On GitLab.com, where things can be slow sometime, we might not even notice that the button has changed color and is inactive. It's as if nothing happened sometimes, while the request is being sent to process the comment and post it.

Proposal

We can add a better visual mechanism to show that the comment is being processed. Suggestions:

• Grey the textarea
• Add a spinning on the button