Replace global js-hidden class with tabs panel modifier

`js-hidden` is a global class that is defined by GOV.UK Template (https://github.com/alphagov/govuk_template/blob/78cbc105b564d2f4a83bb257abb98322bb851a93/source/assets/stylesheets/_accessibility.scss#L17)

This could mean that if users include GOV.UK Frontend into their project it could break the JavaScript fallback
in their project, since the tabs' implementation of `js-hidden` does not check for the presense of the
`js-enabled` class.

To avoid this problem we avoid a global class name and instead create a modifier for the panel component which is being hidden.

src/components/tabs/_tabs.scss

@@ -116,15 +116,15 @@
         border: 1px solid $govuk-border-colour;
         border-top: 0;
 
+        &--hidden {
+          display: none;
+        }
+
         & > :last-child {
           margin-bottom: 0;
         }
       }
     }
 
   }
-
-  .js-hidden {
-    display: none;
-  }
 }

src/components/tabs/tabs.js

@@ -8,7 +8,7 @@ function Tabs ($module) {
   this.$tabs = $module.querySelectorAll('.govuk-tabs__tab')
 
   this.keys = { left: 37, right: 39, up: 38, down: 40 }
-  this.jsHiddenClass = 'js-hidden'
+  this.jsHiddenClass = 'govuk-tabs__panel--hidden'
 }
 
 Tabs.prototype.init = function () {