The "Start page" user preferences page has been reviewed to
consider the new $CFG->enabledashboard setting.
The "Dashboard" won't be displayed in the list if it's disabled.
Besides, the default value is now calculated calling the new
get_default_home_page() method.
Allows display_size to use a fixed unit for easy comparison of
multiple results, and fixed decimal places for the same reason.
Improves behaviour by using consistent decimal places and a
consistent space before the unit (the previous one only has a space
before 'bytes', not before 'KB').
Of existing uses, all the ones that displayed a 'maxbytes' type
configuration setting (which are likely to have an 'exact' size
and would be better shown as 512 KB rather than 512.0 KB) have been
changed to use 0 decimal places, to preserve previous behaviour.
All the uses which were showing an actual file or memory size have
been left as default (1 decimal place).
