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

> Why not using "||" instead of "|"?

I just thought about this some more, and I think it's intentional:

If you do `a || b` and a is true, then b never gets evaluated. In this case, it means that if we successfully replace links in description, then we'll never replace them in steps_to_reproduce, which is not the desired behavior. 

Using the bitwise operator ensures that all replacements are executed.

I'll add a comment to the code to clarify that.