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).
This fixes an issue on those systems that don't fully support unicode
characters within zip structures, which subsequently led to broken
links when browsing downloaded course content.
sam marshall