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/uibinder.js

@@ -145,6 +145,7 @@ async function validateSelectedAccount(){
             setOverlayHandler(() => {
                 document.getElementById('loginUsername').value = selectedAcc.username
                 validateEmail(selectedAcc.username)
+                loginViewOnSuccess = getCurrentView()
                 switchView(getCurrentView(), VIEWS.login)
                 toggleOverlay(false)
             })