Fixed #22778 -- Add an option to define default related_name

Example:

  from django.db import models

  class Author(models.Model):
    name = ...

  class Editor(models.Model):
    name = ...

  class Book(models.Model):
    editor = models.ForeignKey(Editor, related_name="books")
    authors = models.ManyToManyField(Author, related_name="books")

The class Book can now be written :

  class Book(models.Model):
    class Meta:
      default_related_name = "books"
    editor = models.ForeignKey(Editor)
    authors = models.ManyToManyField(Author)
[`create` already saves the model](https://docs.djangoproject.com/en/dev/ref/models/querysets/#django.db.models.query.QuerySet.create), you don't need to repeat it (same for others below)