Bug 26454: Add system preference to set meta description for the OPAC

Bug 26454: Add system preference to set meta description for the OPAC

Websites should have a meta description tag that shows in search engine results.
The content should also be available to set as a system preference.

To test:
1) Look at opac-main source (in the browser) and see that meta name="description" is missing.
2) Apply patch.
3) Run: perl installer/data/mysql/updatedatabase.pl
4) Go to system preferences > OPAC and add a description to OpacMetaDescription.
5) Look at opac-main source and confirm that meta name="description" now has content equal to the text set in the OpacMetaDescription system preference.
6) Sign off.

Sponsored by: Lunds University Libraries
Signed-off-by: Magnus Enger <magnus@libriotech.no>
Works as advertised. Newlines are preserved. Double quote is rendered
as &quot;. Nice enhancement!
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>