Move COMMENT_MODE_FLAT and and COMMENT_MODE_THREADED constants to new CommentMode enum

Problem/Motivation

Part of #3518993: [META] Replace comment constants with enums

Steps to reproduce

Proposed resolution

CommentManagerInterface - COMMENT_MODE_FLAT and COMMENT_MODE_THREADED moves to a CommentMode enum with cases Flat and Threaded.

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet