Added new supported languages

Forward port of 46393980cf from stable/1.7.x
Fixed #22867 -- Memoized django.utils.version.get_git_changeset().

Restored original fix that had to be removed because the old djangoproject
server was still using Python 2.6.
Fixed #23344 -- Removed duplicate docs for file upload settings.
Replaced HAS_SPATIALREFSYS by a database feature
Replaced no_spatialite by connection features

Refs #22632. Thanks Tim Graham for the review.
Replaced no_mysql by connection features

Refs #22632. Thanks Tim Graham for the review.
Updated docs about Spatialite collect/extent support
Fixed #23352 -- Added tests for MigrationGraph.{forwards,backwards}_plan

Test for ValueError raised by MigrationGraph.forwards_plan and
MigrationGraph.backwards_plan.