Add ability to show the migration plan

[Markus Holtermann]

The migrate command already allows one to list all the migrations and their status (applied / not applied) grouped and sorted by app name. Similar to this it would be nice to show the migration plan, i.e. which migration is applied when.

[Markus Holtermann]

There is a pull request at ​https://github.com/django/django/pull/3116

[Markus Holtermann]

As shortly discussed with jax on IRC, having the --list output as part of migrate is somewhat inconsistent. I'd rather add another command showmigrations that offers --list (what manage.py migrate --list is doing right now) as a default and --plan with the ​this output (dependencies are only shown for --verbose 2). We could add a --graph option too that outputs the content for a dotfile, as ​shown in the pull request.

[Markus Holtermann]

I added another PR (​https://github.com/django/django/pull/3153) that adds a dedicated showmigrations command which then also takes care of --list from the migrate command.

[Tim Graham <timograham@…>]

In a1487deebff7bf27a4946a9f5ddd68154fa4834d:

Fixed #23359 -- Added showmigrations command to list migrations and plan.

Thanks to Collin Anderson, Tim Graham, Gabe Jackson, and Marc Tamlyn
for their input, ideas, and review.

[Tim Graham <timograham@…>]

In e08318b4ef25cfe7d1cfbc5ae7d134ee477506ba:

Refs #23359 -- Removed double newline from output of migrate --list

Thanks Berker Peksag for the review.

[Tim Graham <timograham@…>]

In b6e6fcf3:

Refs #23359 -- Removed the migrate --list option per deprecation timeline.

[GitHub <noreply@…>]

In bbf93ef:

Refs #23359 -- Corrected showmigrations help text for the --database option.