Make the `/groups` route behave as expected

The route is supposed to redirect the Groups#index request based on
whether or not a user was logged in. If they are, we redirect them to
their groups dashboard; if they're not, we redirect them to the public
Explore page.

But due to overly aggressive `before_action`s that weren't excluding (or including) the
`index` action, the request always resulted in a 404, whether a user was
logged in or not.

Closes #12660

See merge request !2580