Fixed #21908 -- Added example usage for ModelAdmin.get_inline_instances().

Thanks matt at schinckel.net for the suggestion.
Fixed #21986 -- Added some guidelines for database indexes.

Thanks django-issues at colons.co for the suggestion.
Allowed custom querysets when prefetching single valued relations

The original patch for custom prefetches didn't allow usage of custom
queryset for single valued relations (along ForeignKey or OneToOneKey).
Allowing these enables calling performance oriented queryset methods like
select_related or defer/only.

Thanks @akaariai and @timgraham for the reviews. Refs #17001.
Fixed #16727 -- Added protocol-relative URL support to contenttypes.views.shortcut.
Cleaned up a repr() hack that caused problems on Python3.
Fixed #14549 - Removed restriction of single FKs on intermediary tables