Truncate previews of email messages to ~3 lines

Emails can get very long.

When you’re trying to do other things on the page this results in a lot
of scrolling.

This commit truncates email messages to about 3 lines, and adds a JS
toggle which reveal the full contents of the email.

app/assets/javascripts/expandCollapse.js

@@ -0,0 +1,41 @@
+(function(Modules) {
+  "use strict";
+
+  Modules.ExpandCollapse = function() {
+
+    this.start = function(component) {
+
+      this.$component = $(component)
+        .append(`
+          <div class='toggle' tabindex='0'>...<span class='visually-hidden'>show full email</span></div>
+        `)
+        .addClass('collapsed');
+
+      this.$toggle = this.$component.find('.toggle');
+
+      this.$toggle
+        .on(
+          "click",
+          ".toggle",
+          this.change
+        )
+        .on("keydown", this.filterKeyPresses([32, 13], this.change));
+
+    };
+
+    this.filterKeyPresses = (keys, callback) => function(event) {
+
+      if (keys.indexOf(event.keyCode)) return;
+
+      event.preventDefault();
+      callback();
+
+    };
+
+    this.change = () => this.toggleCollapsed() && this.$toggle.remove();
+
+    this.toggleCollapsed = () => this.$component.toggleClass('collapsed');
+
+  };
+
+})(window.GOVUK.Modules);