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.
Merge pull request #1644 from PaulMcMillan/bump_hash_iterations

Increase default PBKDF2 iterations
Fixed #20702 -- Deprecated get_formsets in favor of get_formsets_with_inlines.

Thanks stanislas.guerra at gmail.com for the report.
Clarified why one must not catch database errors inside atomic.
Fixed #20877 -- added a performance optimization guide
Merge pull request #1463 from evildmp/ticket_20877_work_in_progress

Many thanks to all involved in discussions, and in particular to Tim Graham for painstaking reviews.
Fixed #21133 --Clarifed documentation about strftime formatting.
- missing __init__.py
Merge pull request #1654 from meteozond/patch-1

- missing __init__.py
Addressed inaccuracies in performance docs

* Move the discussion of CachedStaticFilesStorage to the section about
  HTTP. It's really about client-side caching. It doesn't fit with the
  caching utilities from django.utils.functional.

* Tone down the warning against Pypy as per Alex' feedback. It's a valid
  choice for advanced users who are comfortable using a non-standard
  stack.

* Generally reworded the 'Using different versions of available software'
  section.

* Some other minor adjustments to the document.
Merge pull request #1655 from evildmp/ticket_20877_work_in_progress

Addressed inaccuracies in performance docs
Ensured that explicit time zones are rejected by forms.

Refs #19371.
Correct very minor typo

Just changed as to has.
Merge pull request #1656 from mlissner/patch-1

Correct very minor typo
Ensured that BoundField.as_widget always returns properly localized fields.

This is a follow-up to #18777 which improperly converted to strings in
prepare_value and as such caused regressions like #21074.

Refs #18777, #21074
Fixed #21074 -- Added tests for localized datetime fields.

Fields must render values in the current time zone.

This commit only contains tests because this ticket was just a symptom of
a regression from #18777 that was fixed separately.
Fixed #21137 -- Documented best practice for URLconfs with repeated pattern prefixes.
Partial revert of 165f44aa.

That commit didn't always improve readability.

See discussion on django-developers for details.
Removed a few trailing backslashes.

We have always been at war with trailing backslashes.
Fixed a mistake in my previous commit.