Clarified docs for some tags and filters
Fixed #20958 -- Documented that GenericForeignKey fields can't be accessed in forms.

Thanks marky1991.
Fixed #21010 -- Changed ModelState to only copy _meta.local_fields.
Removed "makemigrations --force" from docs since it doesn't actually exist.
Updated six to version 1.4.1.
Tested exc_type instead of exc_value in __exit__.

exc_value might be None even though there's an exception, at least on
Python 2.6. Thanks Thomas Chaumeny for the report.

Fixed #21034.

Forward-port of a8624b2 from 1.6.x.
Fixed #21009 -- Added a versionchanged note regarding changes to clean().

refs [fb1dd6b13a] and [1c4a9bd9ad1]
Fixed #21041 -- Removed a duplicate form in tests.

Thanks tuxcanfly.
Fixed #20900 -- Documented RemoteUserBackend.authenticate
Fixed #21044 -- Documented django.core.urlresolvers.Resolver404

Thanks Keryn Knight for the suggestion.
Merge pull request #1549 from loic/docs

Removed "makemigrations --force" from docs since it doesn't actually exist.
Fixed #20918 -- Tidied up implementation of custom assertions

1. Moved common logic between assertContains and assertNotContains
   into a separate function _assert_contains()

2. Moved common logic between assertTemplateUsed and
   assertTemplateNotUsed into a separate function
   _assert_templateused()

Thanks Marc Tamlyn for the report and review.
Fixed #21000 -- Made cached_db session backend respect SESSION_CACHE_ALIAS
Took advantage of django.utils.six.moves.urllib.*.
Fixed #21047 -- Added CLA mesage on the new contributor advice doc
Fixed #21035 -- Changed docs to treat the acronym SQL phonetically.

The documentation and comments now all use 'an' to
refer to the word SQL and not 'a'.
Fixed #21014 -- Fixed gobbled ImportError in MigrationLoader.
Fixed #21015 -- Fixed MigrationLoader when importlib.import_module returns a file module or an empty directory.
Fixed #16096 -- Added origin attribute to template instances.

Thanks jdunck for the suggestion.
Fixed #20977 -- Fixed writing migrations to disk on Python 3