Bug 28868: Add back mastheadsearch class name

Bug 28868: Add back mastheadsearch class name

In masthead.inc there is a wrapper for the main search bar called #opac-main-search. Prior to Bug 20168 that warpper had a class name of 'mastheadsearch'.

Historically CSS/JS customization of the main search bar was done via the mastheadsearch class.

Test plan:
1. Inscept the element on the OPAC main page and see the element with an ID of 'opac-main-search'.
2. Apply patch
3. Inscept that same element and you should now also see a class of 'mastheadsearch'.
4. Nothing visually should be different with the patch applied.

Signed-off-by: Sally <sally.healey@cheshiresharedservices.gov.uk>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>