Trigger AttributeError in ValidationError.message_dict when error_dict is missing.

The goal of this change is twofold; firstly, matching the behavior of Django 1.6
and secondly, an AttributeError is more informative than an obscure ValueError
about mismatching sequence lengths.

Refs #20867.
Fixed #21573 -- Improved performance of utils.text.normalize_newlines.
Fixed #18531 -- Deprecated Geo Sitemaps

I've chosen a quick deprecation path, as Geo Sitemaps themselves
are deprecated from some time now.
Renamed first argument of class method to cls
Merge pull request #2050 from maurycyp/master

Renamed first argument of class method to cls
Merge pull request #2048 from loic/ValidationError.message_dict

Trigger AttributeError in ValidationError.message_dict when error_dict is missing.
Merge pull request #2052 from loic/setup.cfg

Made flake8 ignore the .git directory.
(Re-)added GeoDjango instructions for building pysqlite2 correctly.

This is a partial undo of 1b142ef5dd.
Updated a bunch of hyperlinks in documentation
Removed an erroneous leading slash introduced by a626bdf648a.
Fixed #21560 -- missing 'slug' field in example code
I updated the documentation, that the modelform now includes the 'slug' field.
Merge pull request #2058 from c-schmitt/fixes_21560

Fixed #21560 -- missing 'slug' field in example code
Fixed E124 pep8 warnings.
Moved imports to the top of the defaultfilters module.
Fix sqlmigrate's output for parameters
Change FKs when what they point to changes
Better default name for migrations we can't give nice names to
Fix altering of SERIAL columns and InnoDB being picky about FK changes
Fixed #21510 -- Readded search reset link in changelist search bar

Thanks moritz.pfeiffer at alp-phone.ch for the report and
Tim Graham for the initial patch.
Fixed #21563 -- Single related object descriptors should work with `hasattr`.

Thanks to Aymeric Augustin for the review and Trac alias monkut for the report.