Thanks Carl Meyer for the report and Tim Graham for the review.

Tests were using an undocumented keyword argument for easily overriding
script prefix while reversing. This is now changed into a test utility
which can be used as decorator or context manager.

Strings are frozen in anticipation of the Django 1.8 release.
Forward port of 1cd2584c from stable/1.8.x

The only one translatable string will be handled in the core catalog.
Forward port of 1bb712d7 from stable/1.8.x

Thanks Pascal Chambon for the initial patch.

Earlier versions of GDAL do not allow the srid to be set to 0,
so it should be a required parameter to ensure compatibility.

- Instantiation of GDALRaster instances from dict or json data.
- Retrieve and write pixel values in GDALBand objects.
- Support for the GDALFlushCache in gdal C prototypes
- Added private flush method to GDALRaster to make sure all
  data is written to files when file-based rasters are changed.
- Replaced ``ptr`` with ``_ptr`` for internal ptr variable

Refs #23804. Thanks Claude Paroz and Tim Graham for the reviews.

Also added display of migration to stdout when verbosity=3.

Fixed create_default_site() to use a default value in case
settings.SITE_ID isn't set; refs #23945.

Refs #24354 -- Prevented repointing of relations on superclasses when migrating a subclass's name change

Forwardport of test and release note from stable/1.7.x

Thanks gbdlin for the initial patch and Tim Graham for the review.

Thanks Max Demars for the suggestion and Tim Graham for the review.