Bug 33317: Add system preference to set meta robots for the OPAC

Bug 33317: Add system preference to set meta robots for the OPAC

Websites must have a robots meta tag to improve search engine crawling,
so we could tell search engines to not index OPAC pages for example.

To test:

1) Look at opac-main source and see that meta name=robots is missing.
2) Apply patch.
3) Run ./installer/data/mysql/updatedatabase.pl
4) Go to system preferences > OPAC and add some directives to
   OpacMetaRobots (ex: noindex,nofollow).
5) Look at opac-main source and confirm that meta name=robots now has
   content equal to the text set in the OpacMetaRobots system
   preference.
6) This should Prevent search engines from indexing Opac pages (if
   OpacMetaRobots is set to noindex)

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>