Start Discussion UX improvement

Creating discussions for merge requests is extremely valuable with their task tracking ability. Creating a discussion is difficult. Either

• Comment on a file (easy, but not everything is file related)
• Type text in the comment field, click the drop down and change to start discussion. (annoying and not discoverable)

We should change the Comment button, to Start Discussion by default. Or at least create two buttons, so that discussions can be created easily.