Improve UX for making resolvable top level comments

The Problem

Per #19949 (closed) (now &4943 (closed)), users wanting to conduct reviews on MRs desired to create a discussion, so that people could reply to their comments in an orderly fashion.

However, the button for creating these comments is dual-purpose and the default is creating a single comment (not resolvable) instead of a discussion thread which creates a resolvable option for reviewing. This resulted in users needing to delete the comment they'd just created and starting again by intentionally choosing the non-default option of "Start thread" so that they had this resolvable option with the same content.

This issue's purpose is to:

• Add "Make Resolvable" checkbox option when creating a new comment to bottom of comment box and when editing an existing top level comment
• Change dual-purpose "Start thread"/"Comment" button to just "Comment"