[Enhancement] Properly redirect on base path and `/login`

[dermalikmann]

On opening the page through its base path (e.g. "https://admin.mymail.server/") the user should be redirected depending if they are logged in.

• If they are, the site should redirect to the same page a user gets send to when successfully logging in (e.g. /manage/directory/accounts)
• If the user is not logged in, the site should redirect to /login.

Currently, on opening the base path, the user gets the login screen, even if logged in, which can be quite confusing. Additionally there should not be two different paths of logging into the website. Have one single login page, served through one single route.

Furthermore, /login should also handle a user that is already logged in, like it handles a user that has successfully logged in (redirecting to e.g. /manage/directory/accounts)