Added support for ST_Difference/ST_Intersection/ST_SymDifference.

This works at least as far back as Django 1.6 according to the test
added in refs #10870.

Thanks Tim Graham for the review.

Thanks to Jonathan Liuti for the report and Tim Graham for the review.

Thanks Collin Anderson and Nick Sandford for work on the patch.

Refs bc0aba77 which dropped
support for SpatiaLite < 2.4.

Since refs #21489, FormSet classes and factories are exposed on the
django.forms package.

Complement to 48548d1a.

Geometry objects have an srid property, so this addition makes the raster api
more similar to the geometries api.

Forwardport of 65bff161 from stable/1.8.x

It doesn't seem relevant for anyone upgrading to Django 1.10
and beyond.

Thanks Daniel Wiesmann for inspiration.

This reverts commit 71ebcb85.

Thanks Andrey Yakovlev for the initial patch.