• manage_custom_field_edit_page.php: use string_attribute() on string
  variables (trying to keep those calls at the location where they are
  displayed so we don't accidentally do it twice or use it somewhere
  where it is uncertain whether it was escaped earlier)
• manage_custom_field_page.php: use string_display() on string variables

git-svn-id: http://mantisbt.svn.sourceforge.net/svnroot/mantisbt/trunk@1894 <a class="text" href="/?p=mantisbt.git;a=object;h=f5dc347c">f5dc347c</a>-c33d-0410-90a0-b07cc1902cb9