Bug 13341 - Hard-coded "Preview" text in OPAC openlibrary.js

Bug 13341 - Hard-coded "Preview" text in OPAC openlibrary.js

The OpenLibrary JavaScript includes an untranslated string, "Preview."
This patch move the string to the template so that it can be translated.

To test, apply the patch and test that the translator picks up the
string:

1. From misc/translator run 'perl translate update [lang]' (e.g. de-DE)
2. Edit misc/translator/po/[lang]-opac-bootstrap.po and add a
   translation for the updated "Preview" string
3. Remove the "#, fuzzy" marker from that entry
4. From misc/translator run 'perl translate install [lang]'
5. Enable the [lang] translation for the OPAC in system preferences
6. Enable the OpenLibraryCovers system preference.
7. In the OPAC switch to the [lang] translation.
7. View the detail page for a title for which there is an OpenLibrary
   cover image. Below it you should see a preview link with the
   translated string you added in step 2.

Works as expected.
Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>