Moved ServerHandler helper class to tests.

It has been only used in the builtin_servers tests since Django 1.4.
Fixed #22421 -- Regression in fixtures loading.

Loading fixtures were failing since the refactoring in 244e2b71f5 for
inheritance setups where the chain contains abstract models and the
root ancestor contains a M2M relation.

Thanks Stanislas Guerra for the report.

Refs #20946.
Fixed #22650 -- Fixed regression on prefetch_related.

Regression from f51c1f59 when using select_related then prefetch_related
on the reverse side of an O2O:

Author.objects.select_related('bio').prefetch_related('bio__books')

Thanks Aymeric Augustin for the report and tests. Refs #17001.
Fixed the ordering of prefetch lookups so that latter lookups can refer to former lookups.

Thanks Anssi Kääriäinen and Tim Graham for the reviews. Refs #17001 and #22650.
Fixed unused import.
Skipped a migrations test that's not supported on MySQL + Python 3.
Fixed #20477: Allowed list of modules for FORMAT_MODULE_PATH

Previously the FORMAT_MODULE_PATH setting only accepted one string (dotted
module path). A feature has been added to also allow a list of strings.

This is useful when using several reusable third party apps that define new
formats. We can now use them all and we can even override some of the formats
by providing a project-wide format module.
Fixed #22639 -- Added missing imports in docs

Added ModelForm and NON_FIELD_ERRORS imports.
Merge pull request #2679 from ramiro/t22421

Fixed #22421 -- Regression in fixtures loading.
Merge pull request #2498 from ramiro/move-serverhandler

Moved ServerHandler helper class to tests.