Fix the comment feature + add to mobile

On the web app, the “add a comment” ability is buggy. The comments sometimes unlink from the text originally highlighted and randomly disappear when new comments are added. I use comments to make notes of edits that need to be made later, so it’s especially frustrating that this simple feature is unreliable.

Additionally, the “add a comment” feature is missing from the mobile app. There have been a dozen times when I’ve gone to add a comment and have had to open the desktop version on my phone’s browser instead.