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)
I'd format it like this to prevent a long line:
````
return self.opts.default_related_name % {
    'class': self.opts.model_name.lower(),
    'app_label': self.opts.app_label.lower(),
}
```