Add a method to remove a user preference
Add a "Reset" button to configuration of homepage portlets

This allows the user to reset to the search's defaults.
Remove HomepageSettings preference on "reset" to pick up global defaults

Ensure that the global settings for "RT at a Glance" are taken when a
user resets their preferences.  Previously, the setting was simply set
to an empty hash, which meant that the global defaults were not applied.

This causes the reset button to revert current session to the current
state of the global configuration, which is thereafter cached for the
duration of the session in $session{'my_rt_portlets'}.
Remove the session cache of the "RT at a Glance" portlet lists

The initial commit of "RT at a Glance" configurability (8b8e90d) added a
caching layer in the session, rather than hit the Attribute for the
preferences on each request.  Remove this caching, in order to ensure
that users who have not overridden the preference always get the most
recent global configuration.  This comes at the cost of an extra 1-2
Attribute queries on the front page, but as these are well-indexed, this
negligable performance impact seems worth it to obey the principle of
least surprise when the admin updates the global configuration.
Tests for user preferences
Merge branch '4.0/reset-rt-at-a-glance' into 4.0-trunk