Improved Query.names_to_path docstring
Fixed #22967 -- Made Model._do_update consistent

Made _do_update behave more strictly according to its docs,
including a corner case when specific concurent updates are
executed and select_on_save is set.
Removed extra enumerate() from Query.setup_joins()
Fixed #23730 -- Moved support for SimpleCookie HIGHEST_PROTOCOL pickling to http.cookie.

This fix is necessary for Python 3.5 compatibility (refs #23763).

Thanks Berker Peksag for review.
Remove Field.get_validator_unique_lookup_type()

Usage of this method was removed pre 1.0 (c2ba59fc).
Fixed #23775 -- Added docs for --bisect and --pair runtests options.

Thanks Baptiste Mispelon for review.
Removed return from __init__.

__init__ isn't allowed to return anything other than None and it isn't
common practice to include a return statement.
Cleaned up the django.template namespace.

Since this module is going to hold both the implementation of the Django
Template Language and the infrastructure for Multiple Template Engines,
it should be untied from the DTL as much as possible within our
backwards-compatibility policy.

Only public APIs (i.e. APIs mentioned in the documentation) were left.
Refactored listing template subdirectories in apps.

This change has the nice side effect of removing code that ran at import
time and depended on the app registry at module level -- a notorious
cause of AppRegistryNotReady exceptions.
Imported BaseEngine from the DEP.

i18n is left aside for now.
Added initial support for loading template engines.
Added dummy template backend.
Added jinja2 template backend.