Change plugin class for comment/user migration to the standard migration plugin class

Problem/Motivation

The core comment migration uses a custom migration plugin class which overrides the process section and uses field plugins to populate the process. This is done in core because the migration is not derived, so we can't add those processes in a deriver like we do with node. This causes problems in the exported migration though, as when people try to customise it (say by overriding the comment_body format) The special plugin class ignores the process section in the migration yml and inserts its own.

It's easily fixed when customizing the migration by changin the class back to Migration, but this isn't intuitive, or documented.

Edit: User does this too. Any others?

Steps to reproduce

Proposed resolution

Switch the class for the comment migration in the deriver, and see if we also need to manually populate the process section with the fields from d7

Remaining tasks

User interface changes

API changes

Data model changes