Fixed #23359 -- Add separate command to list migrations and show migration plan

https://code.djangoproject.com/ticket/23359

This pull request moves the ``--list`` argument from the ``migrate`` command to a new ``showmigrations`` command and additionally adds a ``plan`` option on that one.

The PR can be used instead of #3116 but it needs some doc update (deprecated in 1.8 as well as mentioning in ``docs/ref/django-admin.txt``)
Is this needed? I assume it's to register pre/post_migrate signals but they probably wouldn't be needed for this command?