Bug 8019 - Preserve language choice between Browser sessions

Language choice in Opac an Staff client was not preserved between Browser sessions.
Changed expiry date of cookie 'KohaOpacLanguage' from '' (= expire after session) to '+3y' (3 years from now)

To test:
- In a multilingual Koha istallation, choose a language.
- Log out, close all Browser instances. Go to login screen. Login should show up in same language as before.
- Repeat with other languages.

	modified:   C4/Templates.pm

Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>
Signed-off-by: Paul Poulain <paul.poulain@biblibre.com>

C4/Templates.pm

@@ -300,7 +300,7 @@ sub setlanguagecookie {
     my $cookie = $query->cookie(
         -name    => 'KohaOpacLanguage',
         -value   => $language,
-        -expires => ''
+        -expires => '+3y'
     );
     print $query->redirect(
         -uri    => $uri,