Fixed #12568 -- no error when accessing custom field's descriptor

The SubfieldBase's descriptor caused an AttributeError when accessed
from the class. Introspection didn't like that.

Patch by Trac alias supervacuo.
Fixed #15625 -- Made message in MultiValueDictKeyError less verbose.

Thanks margieroginski for the suggestion.
Fixed #4278 -- Added a dirs parameter to a few functions to override TEMPLATE_DIRS.

* django.template.loader.get_template()
* django.template.loader.select_template()
* django.shortcuts.render()
* django.shortcuts.render_to_response()

Thanks amcnabb for the suggestion.
Fixed #21118 -- Isolated a test that uses the database.

Thanks rmboggs for the report.
Fixed #21098 -- Applied sensitive_post_parameters to MultiValueDict

Thanks simonpercivall for the report and bmispelon for the review.
Fixed #21116 -- Made usage of manage.py in docs more consistent.

Thanks daniel.quattro at gmail.com for the report.
Fixed #19414 -- Added admin registration decorator

Thanks stavros for the suggestion.
Fix #21121: Add archive of security issues.
Merge pull request #1641 from ubernostrum/security-issues-docs-21121

Fix #21121: Add archive of security issues.
Fixed a typo in the docs
Fixed two typos in the docs
Corrected markup problems in new security summary page.
Reworked security issue list to be per-issue, not per-release.
Fixed typo in exception message; refs #19414

Thanks Alexey Boriskin for the report.
Added __pycache__ to gitignore
Fixed #7557 -- Added type checking to Variable initialization.

Thanks tobias for the suggestion and boblefrag and saz for work on the
patch.
Fixed #21125 -- Removed support for cache URI syntax
Fixed #18744 -- Updated docstring to highlight limitations of NamedTemporaryFile

- Noted that this does not allow for reading and writing the same open
file in different processes under Windows.
- Noted that the keyword arguments to NamedTemporaryFile no longer
match the Python version.
Increase default PBKDF2 iterations

Increases the default PBKDF2 iterations, since computers have gotten
faster since 2011. In the future, we plan to increment by 10% per
major version.
Fixed #9532 -- Added min_num and validate_min on formsets.

Thanks gsf for the suggestion.