[CMS] Frontpage should redirect to the login page

The purpose of this task is to hide the Kotahi existing frontpage, as this is being replaced by Flax in Kotahi 2.0.

Frontpage redirect use cases;

1. If a user navigates to a group frontpage they should be redirected to the login page. For example, app.kotahidev.cloud68.co/groupname should redirect to app.kotahidev.cloud68.co/groupname/login
2. An instance with a single group; app.kotahidev.cloud68.co/login you should be redirected to app.kotahidev.cloud68.co/groupname/login
3. An instance with multiple groups; app.kotahidev.cloud68.co/login should redirect to app.kotahidev.cloud68.co