Bug 14189: Use keep_text for the authority header

Bug 14189: Use keep_text for the authority header

includes/authorities-search.inc already used the same behavior but with
another method.
For consistency it's better to have only 1 way to do that.

Test plan:
Go on the authority home page
Fill the input in the header and click on tab 'search all headings' and
'keyword search' and confirm that the text you have filled is kept.

I tested after applying the patch and the text is kept after removing the
part from authorities-search.inc. I also tried applying that jQuery solution
instead of my JS loop but could not get it to work for the other pages with
a reasonable ammount of effort.

Signed-off-by: Jonathan Druart <jonathan.druart@koha-community.org>
Signed-off-by: Eivin Giske Skaaren <eskaaren@yahoo.no>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>