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.
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.