Bug 14764: OPAC news selector

Bug 14764: OPAC news selector

Test plan:

1) Add more than one library, at least two in total is needed. Make note
   of which is your current user's home branch.
2) Go to Tools -> News and add one global ("All") news item and one
   specific for each branch.
3) Apply the patches.
4) Perform database upgrades when you log in.
5) Go to OPAC main, it should work as before: Showing global news as
   well as the news for your home branch once logged in.
6) Go to System Preferences -> OPAC and set 'OPACNewsBranchSelect' to
   yes.
7) Go to OPAC main, you should see the global news item plus any items
   for your home branch for the current user if logged in.
8) Above the news, you will see a dropdown that lists the branches.
   Select one, and click "Change library."
9) You should now see global news and the news for the selected branch,
   regardless of whether logged in or not.

Sponsored-By: Halland County Library
Fixes QA comments:

* Keep the current selected branch selected

* Change OPACNewsBranchSelect -> OPACNewsLibrarySelect

* fix 'system wide only' view for logged in user. We still show system
  wide even when a library is selected.

* Removed the "change library" link

* Added a label to the field.

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Zeno Tajoli <z.tajoli@cineca.it>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>