Upgrade path for Comment 7.x

Problem/Motivation

We need an upgrade path for the Drupal 7 Comment module.

Remaining Tasks

Write migrations (with tests) to handle the following:

• Variables related to comments (e.g., comment_anonymous_page and its ilk; see comment_uninstall() for more) need to be moved into configuration as needed. It looks like D8 Comment maintains no configuration, so this is not needed.
• Comment entities need to be migrated. Only the entity metadata needs to be handled; fields will be dealt with separately in #2416765: Migrate Drupal 7 Field/Instance/View mode settings
• Comment types need to be migrated, with relevant configuration. The Drupal 6 migration path already has migrations for this, and the structures are identical in Drupal 7, so all we need to do is generalize those migrations and their plugins.