Ensure mobile menu works correctly on iOS
Ensure mobile menu works correctly on iOS

- Using an <a> for the mobile menu buttonw without an href
  was causing the mobile menu not to work on iOS devices
- Altered mobile menu to use <button> instead
- Change data-target to use an ID otherwise aria-expanded
  true/false on mobile menu button does not get set correctly

alphagov/govuk_admin_template,alphagov/govuk_admin_template,alphagov/govuk_admin_template