Changed the term 'Bug' to 'Issue', as a step in moving from bug tracker to issue tracker.

git-svn-id: http://mantisbt.svn.sourceforge.net/svnroot/mantisbt/trunk@2500 <a class="text" href="/?p=mantisbt.git;a=object;h=f5dc347c">f5dc347c</a>-c33d-0410-90a0-b07cc1902cb9