D7 comment type migration should emulate Standard

While reviewing #2853872: Migration for forum and article comments: duplicate comment types and incorrect comment_entity_statistics, I realized that Migrate Drupal is creating a new comment type for every single node type that is migrated from Drupal 7. This results in a bunch of cruft, and an extraordinarily confusing migration path for anything related to D7 comments. I discovered that this is largely my fault, as #2532534: Migration Files for Drupal 7 Comments is what introduced the wonky and janky d7_comment_type migration.

Now, I propose to right this wrong. There is no realistic reason for there to be a comment type per node type -- the migration should instead emulate what Standard does, which is...create a single comment type, called 'comment', and a second one for forum posts (which Forum is hard-coded to use, as detailed in #2853872: Migration for forum and article comments: duplicate comment types and incorrect comment_entity_statistics), if there is a 'forum' content type in the D7 source database. This will greatly simplify things and help untangle the Comment migration path from D7.