Fixed #27777 -- Made File.open() work with the with statement

Fixed #27359 -- Made Engine.get_default() return the first DjangoTemplates engine if multiple are defined.

Inadvertently added in db97a884.

Thanks Andrew Brown for the test case.

Regression in 75cf9b5a

Prevented queries from reusing trimmed joins.

Thanks Tim Graham and Markus Holtermann for review.

Thanks Florian Apolloner for contributing to this patch and
Collin Anderson, Markus Holtermann, and Tim Graham for review.

Thanks Morgan Aubert for the test.

Thanks Mariusz Felisiak for review and MySQL advice.

Reverted test changes in a7a7ecd2 and
e43ea36b (refs #27025).

(Fixed #23604 in 2014 and was not added before.)

Previously, the MemcachedCache backend ignored `OPTIONS` and
PyLibMCCache used them to set pylibmc behaviors. Both backends now
pass `OPTIONS` as keyword arguments to the client constructors.

Generated correct admindocs URLs on Python 3. URLs generate 404s on
Python 2, as in older versions of Django.

The fallback logic which allows non-UTF-8 encoded files to be passed to
attach_file() even when a `text/*` mime type has been specified is
moved to attach(). Both functions now fall back to a content type of
`application/octet-stream`.

A side effect is that a file's content is decoded in memory instead of
opening it in text mode and reading it into a string.

Some mimetype-related logic in _create_attachment() has become
obsolete as the code moved from attach_file() to attach() already
handles this.

Workaround a bug with MariaDB and MySQL native client not stripping the
`5.5.5-` prefix.