Fixed #19827 -- Kept stacktrace in defaulttags exception reraising

Thanks Kronuz for the report and the initial patch.
Minor cleanups in templates test imports
Added test runner option to skip Selenium tests (#19854).
Merge pull request #781 from zsiciarz/ticket_19854

Fixed #19854 -- Added test runner option to skip Selenium tests
Fixed #19609: admin Inlines doesn't display help_text for readonly fields

Refactoring: field was renamed to field_name, since flatten_fieldsets returns field name, not field.

Original patch from Marc Aymerich Gubern
Fixed #15877 -- Improved exception when ModelForm has no model class

Thanks theaspect at gmail.com for the report and volrath for the
patch.
Fixed errors introduced in 21f333bcefccc151d6439246f8203d609ab6ca79. Refs #17751
Fixed errors introduced in 664855b74e1f417384dc7aef35a3df5f86db52d6
Changed %r to %s in get_language_info error message.
Proposed fix for #18162.
Merge pull request #791 from khalas/ticket_19811

Changed %r to %s in get_language_info error message
One last final fix for 0ad76843. Tested on py3 and py2
Fixed #19698 -- Cleared sites cache with signals

Thanks ddavies at nomensa.com for the report and the patch and
serdaroncode for reviewing the patch.
Added a draft document explaining how to release Django.

Thanks to James for the first draft; I made a few changes (svn->git) and
some supporting links, but mostly I added FIXME's.
Fixes #17866: Vary: Accept-Language header when language prefix used
Some updates to "how to release Django":

Typo fixes, spell check, some more specifics where possible.
Fixed #19775 - Clarified requirements of the "default" database.

Thanks monkut for the report and wsmith323 for the patch.
Fixed #19880 - Fixed an error in the form wizard initial_dict example.

Thanks almalki for the report.
Merge pull request #796 from ambv/vary

Fixes #17866: Vary: Accept-Language header when language prefix used
Fixed #19579 - Documented that "providing_args" is purely documentational.