[1.7.x] Fixed #22456 -- Replaced 900913 EPSG code by 3857 in some tests

3857 is now the official EPSG code for the Google spherical mercator
projection and some recent versions of GDAL do not recognize 900913
any longer.
Thanks Tim Graham for the report and initial patch.
Backport of c082f3c74c from master.
[1.7.x] Confirmed support for PostGIS 2.1 and GDAL 1.10

Refs #22456.
Backport of 3cea917e3e from master.
[1.7.x] Fixed a KeyError on login with legacy sessions; refs #21649.

Thanks Loic for the report.

Backport of 11e30b684d from master
[1.7.x] Fixed #22444 -- Marked initial SQL/fixture loading as deprecated.

Thanks Karen Tracey for the report.

Backport of a4acb80463 from master