Ability to override comment form heading per comment type

After the introduction of comment types (#2228763: Create a comment-type config entity and use that as comment bundles, require selection in field settings form) it would make a lot of sense to provide the ability to override the comment form headings ("Comments" and "Add new comment") per comment types.

There are two ways we could go about this:

1. Provide theme suggestions to per comment type so that site-builders or themers can override the headings. We could stop here and be happy with that. See #2282205: Revisit comment template suggestions
2. Or provide settings on the comment type config entity, similar to "Submission form settings" for content types where you can configure 1) the list heading (defaults to "Comments") and 2) the form heading (defaults to "Add new comment").