Add page to choose services

Because a user can have multiple services, they need a way to navigate between
them. Normally they can use the ▶ Switcher to do this, except when:

- they first sign in
- they are on a page which isn’t associated with a service (eg user profile) in
  which case we can’t use the switcher because it won’t know what the ‘current’
  service is

So this commit adds a new page with a (fake) list of services.

app/main/views/two_factor.py

@@ -19,6 +19,6 @@ def two_factor():
     if form.validate_on_submit():
         verify_codes_dao.use_code_for_user_and_type(user_id=user.id, code_type='sms')
         login_user(user)
-        return redirect(url_for('.dashboard', service_id=123))
+        return redirect(url_for('.chooseservice'))
 
     return render_template('views/two-factor.html', form=form)