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/__init__.py

@@ -5,5 +5,5 @@ main = Blueprint('main', __name__)
 from app.main.views import (
     index, sign_in, sign_out, register, two_factor, verify, sms, add_service,
     code_not_received, jobs, dashboard, templates, service_settings, forgot_password,
-    new_password, styleguide, user_profile
+    new_password, styleguide, user_profile, choose_service
 )