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`
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.
