Fixed 17478 -- Allowed queryset overriding in BaseModelFormSet init

BaseModelFormSet.forms is now a cached property instead of being
populated in the __init__ method. This behaviour also matches an
example in the documentation.
Thanks Thomasz Swiderski for the report and Simon Charette for the
review.
Fixed #18872 -- Added prefix to FormMixin

Thanks @ibustama for the initial patch and dragonsnaker for opening the
report.
Merge pull request #1294 from LuRsT/added_prefix_to_form_mixin

Fixed #18872 -- Added prefix to FormMixin
Remove EmailField max_length default removal in deconstruct()
Fixed #20587 -- Made convert_values handle None values
Merge pull request #1295 from LuRsT/fix_sqlite_convert_values_20587

Fixed #20587 -- Made convert_values handle None values
Autodetect ForeignKeys and add dependencies/split on circulars
Added missing `versionadded` for `FormMixin.prefix`.
Fixed #20311 -- Make sure makemessages doesn't create duplicate Plural-Forms .po file headers.

Thanks naktinis for the report and initial patch.
Fixed a very old email address of mine still lingering in the AUTHORS file
Fixed errors and inconsistencies in CBV topic documentation.

The code examples should now work correctly.
The `get_context_data` method in the examples was changed when
necessary to adopt a singular style (get context with super(...),
add the extra keys to the dict then return it).

Thanks to Remco Wendt for the initial report and to Tim Graham
for the review.
Fixed #20524  - Described keywords in triaging contrib doc.
Corrected minor typos in FileUploadHandler.receive_data_chunk docs.
Fixed #20643 -- Fixed implementation of JSONResponseMixin in CBV docs

Thanks to Michal Sládek for the report and initial patch,
and to loic84 for the review.
Fixed #20644 -- Add ModelFormMixin.fields to the CBV flattened index

Thanks to Tim Graham for the report and review.
Clarified get_list_or_404 docs, refs #14150.
Fixed #20636 -- Stopped stuffing values in the settings.

In Django < 1.6, override_settings restores the settings module that was
active when the override_settings call was executed, not when it was
run. This can make a difference when override_settings is applied to a
class, since it's executed when the module is imported, not when the
test case is run.

In addition, if the settings module for tests is stored alongside the
tests themselves, importing the settings module can trigger an import
of the tests. Since the settings module isn't fully imported yet,
class-level override_settings statements may store a reference to an
incorrect settings module. Eventually this will result in a crash during
test teardown because the settings module restored by override_settings
won't the one that was active during test setup.

While Django should prevent this situation in the future by failing
loudly in such dubious import sequences, that change won't be backported
to 1.5 and 1.4. However, these versions received the "allowed hosts"
patch and they're prone to "AttributeError: 'Settings' object has no
attribute '_original_allowed_hosts'". To mitigate this regression, this
commits stuffs _original_allowed_hosts on a random module instead of the
settings module.

This problem shouldn't occur in Django 1.6, see #20290, but this patch
will be forward-ported for extra safety.

Also tweaked backup variable names for consistency.

Forward port of 0261922 from stable/1.5.x.

Conflicts:
	django/test/utils.py
Fixed #20653 -- Renamed checksetup management command.

This is to allow future compatibility with work that is ongoing in the 2013 GSoC.
Fixed #20654 -- Fixed type of `m2m_changed`'s `pk_set` arg in docs
Doc cleanup for FormMixin.prefix; refs #18872.