Bug 37257: Enable translation of Copy control in OPAC user summary

Bug 37257: Enable translation of Copy control in OPAC user summary

Currently we define translatable labels for DataTables "export" options
(Copy, CSV, Excel, and Print) in the section of datatables.js for adding
our own configuration to these buttons. This works fine until we
redefine a button without redefining the translatable label as we do in
opac-user.pl.

The correct solution is to define these strings in the default set of
language strings we use for all other DataTable strings. This patch
makes that change in both the staff interface and the OPAC (although the
problem only currently manifests in the OPAC).

To test, apply the patch and start the translation process:

- Install fr-CA or another language in the OPAC
    1. Run the following commands:
        - gulp po:update
        - ./misc/translator/translate update fr-CA
        - ./misc/translator/translate install fr-CA
    2. Go to Administration > System preferences
    3. Search for OPACLanguages and check fr-CA
    4. Set opaclanguagesdisplay to Allow
    5. Click "Save all I18N/L10N preferences"

- Check that "Copy" is translated in the .po files
   1. Open misc/translator/po/fr-CA-messages-js.po
   2. Search for "Copy"
        --> It should be translated as "Copier"

- Log in to the OPAC as a user with checkouts.
- Switch to the language you translated.
- View the user summary page.
  - The controls at the top of the table of checkouts should be
    correctly translated.

- In the staff interface, switch to the language you translated.
- Test any DataTable with export controls, e.g. Administration ->
  Libraries, to confirm that the "Export" menu items are still
  translated correctly.

Sponsored-by: Athens County Public Libraries
Signed-off-by: Caroline Cyr La Rose <caroline.cyr-la-rose@inlibro.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>