Bug 28698: Fix news for all displays in all locations

Bug 28698: Fix news for all displays in all locations

Some news are used for custom text with a specific language, for example "opacheader".
The bug is that in these locations, news for all (staff and opac) are displayed.
This is because GetNewsToDisplay uses SQL :
  opac_news.lang = '' OR opac_news.lang = ?

This patch is a quick fix.
This feature maybe needs a big revamping.

Test plan :
1) Remove any news
2) Create a news with empty 'Display location' and some content
3) Display OPAC interface
4) without patch you see in location of 'opacheader', 'opacnavright' ... the content
   of the news
5) With patch you see content of the news only in news table
6) Create a news for opacheader
7) Check it appears in correct language

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Andrew Fuerste-Henry <andrew@bywatersolutions.com>
(cherry picked from commit 33369d2522c23230e9f755466b19f9c032629365)
Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>