Prevent too many redirects error when home page URL set to external_urll

Many users naively set the home page URL setting to external_url
(e.g. https://mydomain.com). When an unauthenticated user signs in, this
causes endless redirections. For example, this is occuring:

1. Unauthenticated user attempts to access https://mydomain.com/dashboard
2. Application redirects to the home page URL: https://mydomain.com
3. Repeat step 2

In step 3, ApplicationController should have redirected the user to
https://mydomain.com/users/sign_in. Disabling the redirection if home
page URL is the same as external_url prevents users from messing up.

* Closes https://github.com/gitlabhq/gitlabhq/issues/8843
* Closes #2057

See merge request !1210