support geodetic distance calculation on MySQL 5.7+

[Sergey Fedoseev]

​ST_Distance_Sphere function that returns geodetic distance was added in MySQL 5.7.6.

[Tobias Krönke]

I would like to note, that ST_Area also works with MySQL (​https://dev.mysql.com/doc/refman/5.7/en/gis-polygon-property-functions.html#function_st-area), although Django denies its execution:

Region.objects.order_by(RawSQL('ST_Area(polygon)', params=[]))
<QuerySet [<Region: Region object (1)>, '...(remaining elements truncated)...']>

assuming model Region with field polygon = PolygonField(srid=4326, spatial_index=True).

in contrast that errors with contributed Area:

from django.contrib.gis.db.models.functions import Area
Region.objects.order_by(Area('polygon'))
...
django.db.utils.NotSupportedError: Area on geodetic coordinate systems not supported.

[Shinya Okano]

I created the patch ​PR.

Updated the Distance function and be able to use distance lookup on MySQL.

[Tim Graham]

Please uncheck "patch needs improvement" after addressing the test failures and other review comments. If the tests are passing locally, perhaps it's a difference between the versions that Jenkins uses.