Add custom file upload component

The default browser file upload control is difficult to style, but looks totally
out of place.

This commit replaces it with one that has a GOV.UK style button, as a first
step.

Based heavily on this example:
http://tympanus.net/codrops/2015/09/15/styling-customizing-file-inputs-smart-way/

gulpfile.babel.js

@@ -37,6 +37,7 @@ gulp.task('javascripts', () => gulp
     paths.src + 'javascripts/apiKey.js',
     paths.src + 'javascripts/autofocus.js',
     paths.src + 'javascripts/highlightTags.js',
+    paths.src + 'javascripts/fileUpload.js',
     paths.src + 'javascripts/main.js'
   ])
   .pipe(plugins.babel({