Implement Migration.run_before

This attribute (used for reverse dependencies) was previously declared
and mentioned in the code, but never actually used.
Revert "Fixed #19303 -- Fixed ModelAdmin.formfield_overrides on fields with choices"

This reverts commit 9d1987d7679165ad3a7c2b713a8a488cc1421905.
Fixed #22690 -- Added a check for proxy models containing fields.

Removed the FieldError raised by ModelBase.__new__ in this case.
Fixed #22748 -- Corrected post_migrate usage example.

Thanks Rudolph for the report.
Fixed #11776 -- Added CSS class for non-field/top of form errors.

Thanks Daniel Pope for the suggestion.
Fixed #22753 -- Documented ability to use a note with the {% comment %} tag.
Unicode method on model should return unicode

The example of a __unicode__ model method should return a unicode value
instead of a string.
Fixed #22384 -- Deprecated reversing URLs by dotted path.
Merge pull request #2756 from tubaman/return_unicode

Unicode method on model should return unicode
Removed erroneous trailing comma in OLMapWidget.js.
Updated current state of MySQL Connector/Python.
Added a patch review checklist.
Fixed #22681 -- Made TarArchive recognize leading directories properly.
Fixed #22717 -- Auto-corrected missing ending slash in FileSystemStorage

Thanks David Fischer for the report and Moayad Mardini for the
review.
Fixed #22684 -- Amended SelectDateWidget.empty_label to accept a tuple of values.

Thanks danielsamuels for the report
Fixed #13776 -- Fixed ModelForm.is_valid() exception with non-nullable FK and blank=True.

Thanks peterbe for the report.
Fixed #22747 -- Add backwards compatibility tip for new behavior of formset.save(commit=False).

Thanks django at patjack.co.uk.
Fixed #22710 -- Amended aggregation note for empty QuerySets

Thanks smeaton for report and Joël Rochat for spell check
Fixed RST syntax in 1.8 release docs.
Reordered setUp code in prefetch_related tests.

This allows these tests to run on databases that reject duplicate NULL
values in unique fields e.g. SQL Server.

Forward port of 4fb50a9a from stable/1.7.x.