Implement a search link on mobile devices

fionamorrison23 → credited alan.cole → .

fionamorrison23 → credited danielgry → .

fionamorrison23 → credited febdao → .

fionamorrison23 → credited richardgaunt → .

Verified and tested on latest dev version

Testing Results:
1. Verified the search link appears in the header on mobile as configured
2. Validated screen reader behaviour recognises the search button
3. Checked that the search placement is configurable in the Drupal backend

Screenshot: