Bug 14252: (followup) Adds OpacLangSelectorMode syspref

Bug 14252: (followup) Adds OpacLangSelectorMode syspref

Based on discussion, this followup does the following:

1/ brings back the switcher on opac-bottom.inc
2/ adds a syspref OpacLangSelectorMode to toggle between the three
   modes:
     (a) show switcher both on masthead and footer (default)
     (b) footer only
     (b) masthead only

Test plan
=========

1/ apply earlier patches attached to this bug in their correct order
2/ apply this followup patch
3/ run updatedatabase.pl to add in the atomic update
4/ goto admin/preferences.pl?tab=opac
5/ look up OpacLangSelectorMode, it should be set with default value
   "both masthead and footer"
6/ check OPAC to see if both locations show the selectors
7/ change OpacLangSelectorMode to 'only header' and 'only footer' at
   each iteration, and check if the selection has correctly toggled
   the selectors. It should

Note: make sure you do not have the patch 11057 applied on the branch
      from before, otherwise merge conflict might happen.

http://bugs.koha-community.org/show_bug.cgi?id=14252

Followed test plan. Works as expected.
Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Mark Tompsett <mtompset@hotmail.com>
Signed-off-by: Jonathan Druart <jonathan.druart@koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>