Cleanup of manage_customfield*.php

Some modifications elsewhere but only those files have been thoroughly done.

• config_defaults_inc.php:
  (g_custom_field_link_threshold): new option to provide a threshold for users
  who can link and unlink custom fields but not delte, create, or modify them
  (g_custom_field_edit_after_create): new option to control whether a user is
  directed to edit a custom field after creating it

• manage_custom_field_create.php:

  • remove multiple field creation functionality
  • trigger errors properly and immediately
  • check whether to redirect to the edit page or the main custom field page

• manage_custom_field_delete.php:

  • add f_return parameter and redirect there when done
  • don't prevent a user from deleting a used custom field, just make them
    confirm it
  • always confirm deletion of a field

• manage_custom_field_edit_page.php:

  • add f_return parameter
  • use check_checked()

• manage_custom_field_page.php:

  • add a column to list the number of projects in which the field is being used

• manage_custom_field_update.php:

  • add f_return parameter and redirect there when done

• manage_proj_custom_field_add_existing.php:

• manage_proj_custom_field_remove.php:

  • add an additional check to make the user is higher than
    custom_field_link_threshold

• manage_proj_edit_page.php:

  • check custom_field_link_threshold before showing the custom field section
  • check manage_custom_fields before displaying the edit link

• core/custom_field_api.php:
  (custom_field_create): add a check to make sure the field name isn't blank

• core/helper_api.php:
  (helper_ensure_confirmed): add a comment about needing to fix formatting

• lang/strings_english.txt:

  • add new strings for custom field stuff

git-svn-id: http://mantisbt.svn.sourceforge.net/svnroot/mantisbt/trunk@1742 <a class="text" href="/?p=mantisbt.git;a=object;h=f5dc347c">f5dc347c</a>-c33d-0410-90a0-b07cc1902cb9