Replace DefaultSummaryRows preference with SummaryRows

DefaultSummaryRows was, until bestpractical/rt@82c52177, a user override
of the global $DefaultSummaryRows configuration, which was used as the
default value for the SummaryRows _preference_ displayed on
/Prefs/MyRT.html This preference for a default for a preference was
unhelpful, and was removed in the aforementioned commit.

Instead, use the SummaryRows preference's value directly, falling back
to the global $DefaultSummaryRows value if that is unset.  This allows
the /Prefs/MyRT.html setting to take effect for the portlets on the RTIR
homepage.
Merge branch '3.0/homepage-summaryrows' into 3.0-trunk
Revert "Disable and unapply the SLA CF"

This reverts commit a1b70753aee4ded2aa1c1f11819d4bf6236acce8.

Conflicts:
	docs/UPGRADING-3.0
Write some actual SLA upgrade documentation.

The original docs in AdministrationTutorial.pod were likely to be missed
in an upgrade and not super helpful. An early attempt to improve these
docs has been reverted because it would lead to losing track of your SLA
field on upgrade. Unfortunately, it's become clear that when the SLA
code was removed from RTIR, little or not testing was done.
Merge branch '3.0/sla-upgrade' into 3.0-trunk
Merge branch '3.0-trunk' into 3.2-trunk

Conflicts:
	t/upgrade/2.6.0-to-2.9.0.t