Skip ‘choose service’ page if user has one service

We used to do this by redirecting on the choose service page. However when we
lost the dropdown and this page also became the page for adding a new service
(in 3617f2e936) the redirect was removed.

This commit re-adds the redirect on the two factor page, so that it only happens
on first login.

So the flows are:

**Multiple services**
```
`Sign in` → `Enter two factor code` → `Choose service` → `Service dashboard`
```

**One service**
```
`Sign in` → `Enter two factor code` → `Service dashboard`
```

**No services (you’ve deleted all your services)**
`Sign in` → `Enter two factor code` → `Choose service` → `Add new service`

app/main/views/two_factor.py

@@ -5,7 +5,7 @@ from flask import (
 from flask_login import login_user
 
 from app.main import main
-from app.main.dao import users_dao
+from app.main.dao import users_dao, services_dao
 from app.main.forms import TwoFactorForm
 
 
@@ -22,6 +22,7 @@ def two_factor():
     if form.validate_on_submit():
         try:
             user = users_dao.get_user_by_id(user_id)
+            services = services_dao.get_services(user_id).get('data', [])
             # Check if coming from new password page
             if 'password' in session['user_details']:
                 user.set_password(session['user_details']['password'])
@@ -29,6 +30,9 @@ def two_factor():
             login_user(user)
         finally:
             del session['user_details']
-        return redirect(url_for('main.choose_service'))
+        if (len(services) == 1):
+            return redirect(url_for('main.service_dashboard', service_id=services[0]['id']))
+        else:
+            return redirect(url_for('main.choose_service'))
 
     return render_template('views/two-factor.html', form=form)