Fixed #22172 -- Allowed index_together to be a single list (rather than list of lists)..

Thanks EmilStenstrom for the suggestion.
Added newline for flake8.
Fixed #22130 -- fixed template_tests/defaultfilters order dependent test failure
Restored aliases for two APIs removed by app-loading.
Merge pull request #2383 from erikr/fix-tests-fix-ampersands-deprecation

Fixed #22130 -- fixed template_tests/defaultfilters order dependent test failure
Fixed some typos in the documentation.

Thanks to Rodolfo Carvalho and Piotr Kasprzyk
for the patch.
Removed WSGI upgrade instructions for Django 1.3 and earlier.
Edited contrib.admin check messages for grammar and consistency.
Edited contrib.auth check messages for grammar and consistency.
Cleanup of contrib.contenttypes check messages.
Edited contrib.sites check messages for grammar and consistency.
Edited MySQL-specific check messages for grammar and consistency.
Edited model check messages for grammar and consistency.
Edited model and field checks for grammar and consistency.
Added first cut at reference documentation for the checks framework.
Fixed #22193 -- Made hint a truly optional arugment on check messages.

Thanks to Thomas Güttler for the suggestion.
Added backwards compatibliity checks to reference documentation.
Corrected expected test output to allow for cleanups in signal and compatibility checks.
Fixed #22135 -- Added ModelAdmin.get_changeform_initial_data().

Allows custom behavior for setting initial form data in ModelAdmin.
By default, initial data is set via GET params. The new method allows
this behavior to be overridden.

Thanks egasimus for the suggestion.
Fixed many typos in comments and docstrings.

Thanks Piotr Kasprzyk for help with the patch.