Implement geometry validation in GIS database backend

[milosu]

PostGIS provides function ST_MakeValid, that can be used in Django as a PostGISOperator.

There is however no way how to use ST_MakeValid to simply repair GEOS Geometry.

Note that there is also no comparable function in the GEOS library itself (not counting something simple like buffer(0)).

This missing feature is also subject to Stackoverflow questions, like:
​https://stackoverflow.com/questions/45631855/equivalent-of-postgis-st-makevalid-in-django-geos/45633779

Attached patch implements new PostGISOperations method called "make_valid", that can be run from implementation code simply by:

from django.db import connection

repaired_geos_geometry = connection.ops.make_valid(some_goes_geometry)