Add login route

See item 3. of #372 (closed).

I move the login UI under a /login route. Here is the new behaviour:

• If the user does not have a sessionId, render the logged-out routes, that is:
  • a /login route that renders the LoginContainer;
  • a default route that renders a <Redirect /> to /login -- this is how the user gets redirected to the login page after logging out.
• If the user has a sessionId, render the logged-in routes, that is:
  • all the existing routes;
  • a new /login route that renders a <Redirect /> to / -- this is how the user gets redirected to the homepage after logging in.

No dramatic changes, really; it just makes the login flow more transparent for users and the code more robust (i.e. a logged-in component never gets rendered when the user is logged out and vice-versa).