• manage_proj_edit_page.php: don't display the column headers in each section
  when there are no rows to be displayed

• lang/strings_english.txt: use 'Field' instead of 'Fields' so I renamed the
  string

git-svn-id: http://mantisbt.svn.sourceforge.net/svnroot/mantisbt/trunk@1832 <a class="text" href="/?p=mantisbt.git;a=object;h=f5dc347c">f5dc347c</a>-c33d-0410-90a0-b07cc1902cb9