Bug 15039: Move top language menu to include and streamline logic

Bug 15039: Move top language menu to include and streamline logic

The language menu in koha-tmpl/opac-tmpl/bootstrap/en/includes/masthead.inc is used at other places as well (see Bug 14776).

This patch moves it to a file masthead-langmenu.inc to make it re-usable.

Additionally it streamlines the logic (not all combinations of Opac sysprefs that should display the menu did so).

To test:
- Apply patch
- Verify, that the language menu displays / does not display with combinations of:
  - opaclanguagedisplay (Allow)
  - opaclanguages ( > 1 language selected)
  - OpacLangSelectorMode (top or both)
- Verify that sysprefs opacuserlogin and EnableOpacSearchHistory do not interfere
  with the lenguage menu (before, at least one of them had to be on to display the menu)
- Verify that language switching works as before.

Signed-off-by:  Chris Kirby <christopherlawrencekirby@gmail.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>