Hide exporting of password file from students.

EduNetworkBuilder/PersonProfileForm.cs

@@ -330,6 +330,7 @@ namespace EduNetworkBuilder
             if (CurrentUser == null) return;
             btnGenerateStudentFiles.Visible = false;
             btnImportStudents.Visible = false;
+            btnExportStudentPasswords.Visible = false;
             dtpHWDue.Enabled = false;
             TreeNode Top = CurrentUser.WorkTree();
             if (Top != null)