Add API key component

This commit adds a component for showing an API key. Usage:

```jinja
{{ from 'components/api-key.html' import api_key }}
{{ api_key('e1b0751388f3cd0fc9982c701acdb3c2') }}
```

Depending on the user’s browser, it works in three different ways.

No Javascript
---
The API key is shown on the page.

Older browsers with Javascript
---
The API key is hidden, and users can click a button to reveal it.

Newer browsers that support copying to clipboard without Flash
---
As above, but when the key is shown there is a button which copies it to the
clipboard. This is acheived by using
[this polyfill](https://www.npmjs.com/package/query-command-supported)
to reliably detect browser support for the ‘copy’ command.

The styling of the component is a bit different to the initial sketch. I think
a grey button works better than green. Green feels like it’s going to take you
somewhere else.

gulpfile.babel.js

@@ -34,17 +34,19 @@ gulp.task('javascripts', () => gulp
   .src([
     paths.npm + 'govuk_frontend_toolkit/javascripts/govuk/modules.js',
     paths.npm + 'govuk_frontend_toolkit/javascripts/govuk/selection-buttons.js',
-    paths.src + 'javascripts/highlightTags.js',
+    paths.src + 'javascripts/apiKey.js',
     paths.src + 'javascripts/dropdown.js',
+    paths.src + 'javascripts/highlightTags.js',
     paths.src + 'javascripts/main.js'
   ])
   .pipe(plugins.babel({
     presets: ['es2015']
   }))
   .pipe(plugins.uglify())
-  .pipe(plugins.addSrc.prepend(
-    './node_modules/jquery/dist/jquery.min.js'
-  ))
+  .pipe(plugins.addSrc.prepend([
+    paths.npm + 'jquery/dist/jquery.min.js',
+    paths.npm + 'query-command-supported/dist/queryCommandSupported.min.js'
+  ]))
   .pipe(plugins.concat('all.js'))
   .pipe(gulp.dest(paths.dist + 'javascripts/'))
 );