[1.7.x] Added RemoteUserMiddleware to middleware reference page.

Backport of 465980d070 from master
[1.7.x] Added a note regarding django.contrib.auth.authenticate.

Thanks Josh Brown for the suggestion.

Backport of ae5b662796 from master
[1.7.x] Fixed #22453 -- Corrected DoesNotExist message in docs.

Thanks Eugenio Cano-Manuel.

Backport of eefad142de from master
[1.7.x] Fixed spelling errors in documentation.

Backport of b5ce21accb from master
[1.7.x] Fixed #22457 -- Fixed contributing guide w/o github set-up

The published commnand was not working when the user did not have
the git client set-up with the public-key. Changed the contributing
guide to clone it from https instead.

Backport of 28102991172ab931d030412275f926c1b05c688d from master.
[1.7.x] Fixed #22460 -- Explicity remove constraints when dropping a related field.

Backport of 0e45669fa9 from master
[1.7.x] Fixed #22455 -- Documented return values for send_mail and send_mass_mail

Backport of 7cd74fc96ee982acfed7a98e76d4623eabde146d from master.
[1.7.x] Fixed #22434 -- Retain ordering on related sliced subqueries.

Thanks maciej.pawlisz for the report, and charettes for the review.

Backport of a13df671a5 from master
[1.7.x] Fixed #22220 -- Added more examples to reverse() documention.

Thanks EvilDMP for the suggestions.

Backport of 030dd4f72c from master