Fixed #23167 -- Added BaseForm.__repr__()

Thanks Keryn Knight for the idea.
Tweaked docstrings from previous commit.
Fixed #23333 -- Made urlsafe_base64_decode() return proper type on Python 3.
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
Clarified release preparation step about dates.
Fixed #23341 -- Added migration name to nonexistent migration error in makemigrations.
Fixed #22154 -- Added "Built-in" to some class-based views titles.
Fixed #21648 -- Deprecated is_admin_site option to auth.views.password_reset().
Fixed #23131 -- Documented serialization of callable functions in migrations.
Fixed #23355 -- Updated admin jQuery to 1.11.1.
Alphabetized AUTHORS by first name.
Removed unnecessary absolute_imports.
Made main user disconnect after creating test user/tablespaces on Oracle