Added UI and implementation for the account settings tab.

Features:
* Add a new account.
* Switch accounts.
* Log out of an account.

Also added a cancel button to the login UI which is only shown when a user is adding an account. In that case, the operation should be and is cancellable.

app/assets/js/scripts/landing.js

@@ -116,6 +116,7 @@ document.getElementById('launch_button').addEventListener('click', function(e){
 
 // Bind settings button
 document.getElementById('settingsMediaButton').onclick = (e) => {
+    prepareSettings()
     switchView(getCurrentView(), VIEWS.settings)
 }