Prevent doubling clicking form submissions

Have seen users complaining that they got an invitation email twice.
This is probably because they clicked the button twice even though they
think they only clicked it once.

Double form submission is a common issue on web pages, and there are a
number of different ways to prevent it. I’ve chosen to do it this way
because:
- temporarily, not permanently disabling the button means that this
  addresses the double clicking issue without breaking things if the
  user did, really want to click the button again deliberately (for
  whatever reason)
- doing it with a `data` attribute, rather than the `disabled` attribute
  means that the interaction behaviour of the button doesn’t change (
  `disabled` buttons can’t be focused, for example)

app/assets/javascripts/preventDuplicateFormSubmissions.js

@@ -0,0 +1,30 @@
+(function() {
+
+  "use strict";
+
+  let disableSubmitButtons = function(event) {
+
+    $submitButton = $(this).find(':submit');
+
+    if ($submitButton.data('clicked') == 'true') {
+
+      event.preventDefault();
+
+    } else {
+
+      $submitButton.data('clicked', 'true');
+      setTimeout(renableSubmitButton($submitButton), 1500);
+
+    }
+
+  };
+
+  let renableSubmitButton = $submitButton => () => {
+
+    $submitButton.data('clicked', '');
+
+  };
+
+  $('form').on('submit', disableSubmitButtons);
+
+})();

gulpfile.babel.js

@@ -67,6 +67,7 @@ gulp.task('javascripts', () => gulp
     paths.src + 'javascripts/listEntry.js',
     paths.src + 'javascripts/liveSearch.js',
     paths.src + 'javascripts/errorTracking.js',
+    paths.src + 'javascripts/preventDuplicateFormSubmissions.js',
     paths.src + 'javascripts/main.js'
   ])
   .pipe(plugins.prettyerror())