Bug 18084: Language selector hidden in user menu on mobile interfaces

Bug 18084: Language selector hidden in user menu on mobile interfaces

This patch updates templates and CSS so that the language-selection menu
in the header will not be hidden if the browser width is too narrow.

This patch also makes some more general improvements to the responsive
adaptability of the header elements.

To test, apply the patch and rebuild the OPAC CSS
(https://wiki.koha-community.org/wiki/Working_with_SCSS_in_the_OPAC_and_staff_client).

Test the changes by viewing pages in the OPAC and adjusting your browser
width, paying attention to how elements in the top menu adapt to
different sizes. Test under varying conditions:

 - Logged in/not loggeed user
 - Search history enabled and disabled
 - OpacLangSelectorMode showing menu in header or not
 - opacuserlogin enabled or disabled

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>