PEP 8 cleanup
Fixed #19080 -- Fine-grained control over select_related in admin
Fixed #20564 -- Generic relations exclude() regression

The patch for #19385 caused a regression in certain generic relations
.exclude() filters if a subquery was needed. The fix contains a
refactoring to how Query.split_exclude() and Query.trim_start()
interact.

Thanks to Trac alias nferrari for the report.
Merge pull request #1245 from oinopion/list_select_related

Fixed #19080 -- Fine-grained control over select_related in admin
Fixed #20502 (again) -- More i18n cache flush in tests

Thanks Timo Graham for noticing the failures.
Fixed #12337 - Honor ModelForm.Meta.exclude when saving ManyToManyFields.

Thanks margieroginski for the report.
Bumped verion numbers for 1.6a1.
Explained that timezone.now() always returns times in UTC.

The docs were ambiguous about the time zone for now(), leading people to
assume that it would be the current time zone rather that UTC.
Fixed #19925 - Added validation for REQUIRED_FIELDS being a list

Thanks Roman Alexander for the suggestion.
Update submitting-patches.txt

Few quick fixes to the terminology in order to match what is used on the Django Trac at the moment.
Merge pull request #1254 from russkel/patch-1

Updated submitting-patches.txt
Allow build of translated documentation
Update flatpages.txt

Fixed typo
Update media.txt

Fixed typo: froms -> forms
Replaced a dict emulating a set with a set.
Merge pull request #1256 from krmboya/patch-1

Fixed #20575 -- Typo in docs/topics/forms/media.txt
Fixed #20567 - Documented BoundField.id_for_label.

Thanks littlepig for the suggestion.
Fixed #19875 - Added warnings regarding DEBUG=False and empty ALLOWED_HOSTS
Added runserver validation to detect if DEBUG=False and ALLOWED_HOSTS is empty.

Refs #19875.
Refactored file_storage tests to avoid depending on servers tests.