XML Import/Export plugin: bug fixes and improvements 

This pull request covers the following issues

- [0017774](http://www.mantisbt.org/bugs/view.php?id=17774): XML import plugin does not process links
- [0017775](http://www.mantisbt.org/bugs/view.php?id=17775): XML import plugin does not replace links in 'steps to reproduce'
- Fix incorrect default value for due_date in BugData object
- HTML/CSS fixes for 1.3 branch
- Code cleanup