webadmin: fix client sorting for size columns

Currently, size columns are sorted using LexoNumericComparator,
which is fine for most cases except for '< 1 GB' value...
Hence, overridden 'makeSortable' method to compare the size
values numerically.

Screenshot: http://i.imgur.com/KfJjdq8.png

Change-Id: Iab8cdc171ff8f652ec0747212f6fd846cf95540e
Bug-Url: https://bugzilla.redhat.com/1133264
Signed-off-by: Daniel Erez <derez@redhat.com>