Bug 8019 - Preserve language choice between Browser sessions

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>