Custom fields don't allow sorting without lucene

This time it's a UI issue. The issuetable.vm checks for:

#if ($sortingenabled == true && $field.sorter && $field.sortComparatorSource

it shouldn't really check for field.sorter.

Also need to change the default NavigableField to return null sortComparatorSource when the sorter is null.

Test this using the JIRA Toolkit with the Days since last commented field