Disambiguate migrations by group name

Follow-on to #1839644: Extending group objects...

Now I'm getting more ambitious - it's pretty ugly to prepend group names to migration machine names to make sure they're unique, when we now have a group_name column - I may change the PK of migrate_status to include both machine_name and group_name, so we can use the group name only when necessary for disambiguation. E.g., in the wordpress_migrate scenario, currently we may have MyBlog as a group name and MyBlogAuthor, MyBlogBlogPost, etc. as migration machine names - with this change you would have Author and BlogPost as machine names, and could run migrations off those as-is if they're unique. If you import a second blog, MyOtherBlog, then in drush you would have to run MyBlog.Author vs. MyOtherBlog.Author.