Properly app_template_dirs when INSTALLED_APPS change.
Don't try to load app directory templates from apps with a path (eg eggs)
Created a constant for the migrations module name.

Mostly for consistency with MODELS_MODULE_NAME; it's unlikely to change.
Fixed #21650 -- Corrected bad advice for plural translation.

Thanks nedbatchelder and claudep.
Fixed a couple flake8 warnings.
Refactored dumpdata with app configs instead of models modules.
Fixed #21692 -- Quote table name when creating it.
Avoided %r formatting on possibly unicode strings.

The u prefix looks bad on Python 2.
Merge pull request #2120 from ramiro/schema-creation-quoting

Fixed #21692 -- Quote table name when creating it.
Amend test table name so it doesn't clash.

Refs #21692 and 3efd1b8b939.