Bug 23276: Do not display tag if pref TagsEnabled is off

Bug 23276: Do not display tag if pref TagsEnabled is off

If the pref TagsEnabled is off we should not display the tags at the
OPAC.
There is a message to tell that tags system is disabled, but the tags
are displayed.

We should redirect to 404 like we do in opac-topissues.pl and
opac-suggestions.pl.

Test plan:
- Turn TagsEnabled on
- Add some tags
- Turn TagsEnabled off
- Hit /cgi-bin/koha/opac-tags.pl
=> Without this patch you see a warning messaging saying that the tag
system is disabled, but the tags are displayed
=> With this patch you get a 404 redirect

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>