Bug 11993: fix untranslatable confirm popup in request.tt

This patch makes confirm popup text translatable. It implements a
similar function as used on OPAC deleteConfirm() and a JS var message.

This function, or variants, can also be found on other files on intranet,
I think that the only valid places are staff-global.js and help-top.inc

Redefinitions of this function:
prog/en/modules/serials/subscription-frequencies.tt:6
prog/en/modules/tools/marc_modification_templates.tt:158
prog/en/modules/virtualshelves/shelves.tt:92

Last case is a little different

To test:

1) Confirm that the "cancel hold" link correctly prompts for
   confirmation:
   a) Place an item-level hold on a title.
   b) Check in the item and confirm the hold.
   c) Return to the place hold screen for that title and submit another
      patron to place a hold for.
   d) On the place hold screen for that patron, look at the table of
      items. There should be a "cancel hold" link next to the item for
      which the hold was confirmed in step (b).
   e) Click "cancel hold."  You should see a confirmation message.
      Cancelling this dialog should cancel the operation. Confirming it
      should cancel the hold.

2) Check the string is not present on staff PO file
3) Apply the patch
4) Update translation files (cd misc/translator; perl translate update
   xx-YY)
5) Verify the string is now present

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>