Added example of using sitemaps with static views.

References #16829.
Fixed a rest mistake I introduced in d5ce2ff.
Merge pull request #1098 from zsiciarz/ticket-16829

Added example of using sitemaps with static views.
Fixed #12674 -- provide a way to override admin validation

Moved admin validation code to classes and have those be class
attributes to the ModelAdmin classes.
Fixed #11915: generic Accept-Language matches country-specific variants