Allow disabling session model database table creation if not used by session engine

[Tim Graham]

Previously, we recommended removing contrib.sessions from INSTALLED_APPS but this causes the test client login to break (#22934) and also disables the clearsessions management command which could be useful for the file-based sessions (which also don't require the database tables).

[Tim Graham <timograham@…>]

In 3a85aae2eacae836e2a92de77615569883b81932:

Revert "Improve cookie based session backend docs." refs #20418

This reverts commit d9c01da1f8ba3d4e0947fd1d619c8f4a9b013360.

This is poor advice as it breaks the test client login (refs #22934).
We can add a note like this back after refs #22986 is resolved.

[Tim Graham <timograham@…>]

In a3d710a38f1fab5e092cef7a17826674a00e70ea:

[1.7.x] Revert "Improve cookie based session backend docs." refs #20418

This reverts commit d9c01da1f8ba3d4e0947fd1d619c8f4a9b013360.

This is poor advice as it breaks the test client login (refs #22934).
We can add a note like this back after refs #22986 is resolved.

Backport of 3a85aae2ea from master

[Tim Graham <timograham@…>]

In f3bdb83ba58a696c0696d8a6706d2c44a00da68a:

[1.6.x] Revert "Improve cookie based session backend docs." refs #20418

This reverts commit d9c01da1f8ba3d4e0947fd1d619c8f4a9b013360.

This is poor advice as it breaks the test client login (refs #22934).
We can add a note like this back after refs #22986 is resolved.

Backport of 3a85aae2ea from master

[Nick Sandford]

I was going to tackle this, and thought that maybe we could abuse _meta.swappable, but it appears that ModelBase._check_swappable actually checks that the swappable model exists.

I wonder if this would be a use-case for the app registry to allow unregistering models? Then in the session app config's ready() we could conditionally unregister the Session model if the session engine doesn't require it.

My approach was to override AppConfig.get_model and AppConfig.get_models as a way to "disable" the session model. Unfortunately this doesn't work for migrations.

PR here: ​https://github.com/django/django/pull/2960

[Chris Jerdonek]

I haven't looked into the history of this, but it seems like one of the issues is that INSTALLED_APPS is currently being used for two different but related purposes: 1) to say which apps are being used, and 2) to say what models need to be added to the schema. I wonder what would be the best way to make this distinction.

[Aymeric Augustin]

Since Django 1.7 we have a clean way to prevent the creation of database tables, which is the crux of the issue here:

MIGRATION_MODULES = {
    'sessions': None
}

On one hand, it's a slightly convoluted way to declare that you aren't using the Session model. On the other hand, it's a reasonable way to tell Django not to run migrations that create the table backing the Session model.