Add composite PK admin support

[Csirmaz Bendegúz]

This is a follow up to #373 (CompositePrimaryKey).

My proposal is to separate the primary key fields with a comma e.g. /admin/posts/post/foo,bar/change.

The comma is not escaped when used as a separator, but it is escaped (%2C) when part of the primary key's value.
e.g. pk = ("The,quick", "brown") -> /admin/posts/post/The%2Cquick,brown/change

According to ​RFC 3986, unesacped commas are allowed in URI paths (as far as I understand).

What if an existing, non-composite primary key contains a comma?
While Django URL encodes commas in the admin URLs (see QUOTE_MAP), it must be able to look up objects by non-encoded PKs too (see #12349, #18550).
The solution is simple. We must check if the model the user is trying access has a composite primary key or not.

Since admin's LogEntry model uses "generic foreign keys", this ticket depends on #35941 (composite GenericForeignKeys).

[Sarah Boyce]

PR: ​https://github.com/django/django/pull/18865

Marking as "Needs improvement" due to the dependency on #35941

[Csirmaz Bendegúz]

Yes that's correct, thanks Sarah