Bug 26647: Add translation context to cancel hold button in OPAC

Bug 26647: Add translation context to cancel hold button in OPAC

0) Apply the patch and add a hold on an item.
1) Install Swedish language, sv-SE and change interface language to
   Swedish
2) Modify sv-SE-opac-bootstrap.po and add this:
   msgctxt "Cancel hold button"
   msgid "Cancel"
   msgstr "Ta bort"
3) Go to the Holds table at /cgi-bin/koha/opac-user.pl#opac-user-holds
4) Confirm that the button is translated as "Ta bort"
5) Sign off

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>