Re-enable client-side handling of the Via link

This avoids a redirect in the common case that JS is
available and has loaded.

 * Refactor the controller setup for the home route
   to be consistent with how it is handled for other pages.

h/static/scripts/installer-controller.js

@@ -29,6 +29,16 @@ function showSupportedInstallers(rootElement) {
   showIf('.js-install-any', offerChromeInstall || offerBookmarkletInstall);
 }
 
-module.exports = {
-  showSupportedInstallers: showSupportedInstallers,
-};
+function InstallerController(element) {
+  showSupportedInstallers(element);
+
+  // setup Via link form
+  var proxyForm = document.querySelector('.js-proxy-form');
+  proxyForm.addEventListener('submit', function (event) {
+    event.preventDefault();
+    var url = proxyForm.elements['url'].value;
+    window.location.href = 'https://via.hypothes.is/' + url;
+  });
+}
+
+module.exports = InstallerController;

h/static/scripts/site.js

 var CreateGroupFormController = require('./create-group-form');
 var DropdownMenuController = require('./dropdown-menu');
+var InstallerController = require('./installer-controller');
 var ShareGroupFormController = require('./share-group-form');
 
-var installerController = require('./installer-controller');
-
 // load our customized version of Bootstrap which
 // provides a few basic UI components (eg. modal dialogs)
 require('../styles/vendor/bootstrap/bootstrap');
@@ -23,7 +22,7 @@ document.addEventListener('DOMContentLoaded', function () {
   // setup route
   var route = document.location.pathname;
   if (route.match('^/(new-homepage)?$')) {
-    installerController.showSupportedInstallers(document);
+    new InstallerController(document.body);
   } else if (route.match('^/groups') === 0) {
     setupGroupsController(route);
   }

h/static/scripts/test/installer-controller-test.js

@@ -11,7 +11,8 @@ describe('installer page', function () {
     rootElement.innerHTML =
       '<button class="extension js-install-chrome is-hidden"></button>' +
       '<button class="bookmarklet js-install-bookmarklet is-hidden"></button>' +
-      '<input class="link">';
+      '<input class="link">' +
+      '<form class="js-proxy-form"><input name="url"></form>'
     extensionBtn = rootElement.querySelector('.extension');
     bookmarkletBtn = rootElement.querySelector('.bookmarklet');
     linkField = rootElement.querySelector('.link');
@@ -23,11 +24,10 @@ describe('installer page', function () {
   });
 
   function createController(userAgentInfo) {
-    var controller = proxyquire('../installer-controller', {
+    var Controller = proxyquire('../installer-controller', {
       './ua-detect': userAgentInfo
     });
-    controller.showSupportedInstallers(rootElement);
-    return controller;
+    return new Controller(rootElement);
   }
 
   function isHidden(el) {