Added "Switch user" functionality.

The user isn't logged out until the new user logs in. This allows you to
click on "Log in as different user", hit the back button, and still be
logged in.

src/templates/layout.jade

@@ -299,6 +299,11 @@ html(lang="en")
 											| Subscription
 									li.divider(role="separator")
 									| {% endif %}
+									li
+										a.navbar-item(
+											href="{{ url_for('users.switch') }}")
+											i.pi-log-out(title="Log Out")
+											| Not {{ current_user.full_name }}? Log in as another user
 									li
 										a.navbar-item(
 											href="{{ url_for('users.logout') }}")