Clearer explanation when exception has no message

"No exception supplied" is misleading; actually there is an exception,
but there's no message string.
Fixed #18709 -- Check if initial_value is a callable

In _get_changed_data, check if initial_value is a callable and call it
if it is.
Fixed #19895 -- Second iterator over invalid qs raises errors

The fix was already in the patch for #18702, so only test added for
ticket #19895.
Merge pull request #246 from dekkers/ticket_18709

Fixed #18709 -- Check if initial_value is a callable
Fixed #20463 -- Made get_or_create more robust.

When an exception other than IntegrityError was raised, get_or_create
could fail and leave the database connection in an unusable state.

Thanks UloPe for the report.
Fixed #20478 – Added support for HTTP PATCH method in generic views.
Fixed a broken link introduced in a542b808baf.
Fixed #17048 - Added docs for upgrading Django.

Thanks Donald Stufft for the suggestion and
Susan Tan and Brian Fabian Crain for the patch.
Added a note about pip upgrade flag.

Thanks Jannis Leidel for the suggestion.
Added some text I meant to add in the last commit.
Clarified policy for stable branches.

Thanks Ramiro Morales for the initial patch and
Preston Holmes for the review.
Fixed #19237 -- Used HTML parser to strip tags

The regex method used until now for the strip_tags utility is fast,
but subject to flaws and security issues. Consensus and good
practice lead use to use a slower but safer method.
Removed check for 0.15 version of gettext tools

gettext 0.15 has been released in July 2006.
Merge pull request #1198 from KrzysiekJ/http-patch-method

Fixed #20478 – Added support for HTTP PATCH method in generic views.
Fixed #11398 - Added a pre_syncdb signal
Merge pull request #1200 from dstufft/pre-syncdb-signal

Fixed #11398 - Added a pre_syncdb signal
Fixed comments for translators example in the i18n documentation

The documentation regarding adding a comment for translators was confusing
as the example contained mismatching tags.
Merge pull request #1201 from takis/docs_typo

Fixed comments for translators example in the i18n documentation
Made fix for #9321 less buggy and more effective.

Don't try to be smart about building a good-looking help string
because it evaluates translations too early, simply use the same old
strategy as before. Thanks Donald Stufft for the report.

Also, actually fix the case reported by the OP by special-casing
CheckboxSelectMultiple.

Added tests.

Refs #9321.
Fixed #19237 (again) - Made strip_tags consistent between Python versions