Fixed #19945 -- Fixed default User model Meta inheritance.

Updated default User model Meta class to extend AbstractUser Meta
where translated verbose_name and verbose_name_plural are
defined.
Removed unnecessary imports.
Fix Typo explicitly. Fixes #19971
Fixed #19942 -- Decoded errors coming from Popen commands

Thanks Aymeric Augustin for reporting the issue.
Merge pull request #877 from tcarlander/master

Fixed #19971 -- Typo in CBV docs.
Fixed #19915 - Made blocktrans tag honor TEMPLATE_STRING_IF_INVALID.

Thanks Natalia Bidart for the report and Matías Bordese for the fix.
Release notes blurb for 804366327d728d23a9f7a25ff77a6eed3c9f9323.
Removed mentions of regressiontests.
Removed mentions of modeltests.
Merge pull request #871 from matiasb/ticket_19945

Fixed #19945 -- Fixed default User model Meta inheritance.
Fixed #19464 -- Eased customization of ClearableFileInput's link markup

Thanks rubyruy for the report and the patch.
Fixed #19838 -- Admin: Don't leak a 500 HTTP status when trying to delete protected FKs.

Thanks rafadev for the report and Javier Mansilla for the fix.
Fixed #6585 -- Admin relationship widgets: Respect ordering defined by target model's ModelAdmin.

Thanks Gary Wilson for the report and Juan Pedro Fisanotti, Carlos
Matías de la Torre for the fix.