[1.5.x] Fixed #19960 -- Fixed sentence in contrib.auth signals docs

Thanks edd at slipszenko.net for the report.
Backport of 6d3b0c33dc from master.
[1.5.x] Fixed #19962 - Added a note about SESSION_EXPIRE_AT_BROWSER_CLOSE and browsers that persist sessions.

Thanks David Sanders.

Backport of fe5d9fe5fe from master
[1.5.x] Fixed #19933 -- Catched ValueError in get_system_username

Thanks lanre.hbs at gmail.com for the report.
[1.5.x] Fixed #20008 -- Removed trailing slash in Wikipedia link

Thanks senden9 at gmail.com for the report.
Backport of 63c52dcbcd from master.
[1.5.x] Fixed #19965 - Added a warning that the tutorial is written for Python 2.

Thanks itsallvoodoo for the patch.

Backport of e1bafdbffa from master.