Refs b3acdeee.

Thanks to Josh Smeaton for the idea of implementing server-side cursors
in PostgreSQL from the iterator method, and Anssi Kääriäinen and Kevin
Turner for their previous work. Also Simon Charette and Tim Graham for
review.

Regression in b52c7300.

Thanks Morgan Aubert for the test.

After refs #16614, integers might appear outside the WHERE clause.

The template-based widget rendering branch was started years ago.
This is obsolete now.

Without this, tests from refs #27432 fail when running with --reverse.

While our code also works with any Jinja2 >2.7,<2.9 use >2.9.2 for
simplicity reasons in the requirements file.

Kept the build green until a regression is fixed:
https://github.com/pallets/jinja/issues/640

Thanks Harris Lapiroff for the report.

Removed the trailing newline from widget attrs.html template.
The solution may be revisited by fixing refs #9198 but not
for Django 1.11.

Thanks Dmitry Ivanchenko for the report and Preston Timmons for advice.

Uncertain if this is needed, but no failures have appeared so far.

Thanks Pavel Pokrovskiy for the report and Tim Graham for the review.