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 the
`index` action, the request always resulted in a 404, whether a user was
logged in or not.

Closes #12660

app/controllers/groups_controller.rb

@@ -2,17 +2,18 @@ class GroupsController < Groups::ApplicationController
   include IssuesAction
   include MergeRequestsAction
 
-  skip_before_action :authenticate_user!, only: [:show, :issues, :merge_requests]
   respond_to :html
-  before_action :group, except: [:new, :create]
+
+  skip_before_action :authenticate_user!, only: [:index, :show, :issues, :merge_requests]
+  before_action :group, except: [:index, :new, :create]
 
   # Authorize
-  before_action :authorize_read_group!, except: [:show, :new, :create, :autocomplete]
+  before_action :authorize_read_group!, except: [:index, :show, :new, :create, :autocomplete]
   before_action :authorize_admin_group!, only: [:edit, :update, :destroy, :projects]
   before_action :authorize_create_group!, only: [:new, :create]
 
   # Load group projects
-  before_action :load_projects, except: [:new, :create, :projects, :edit, :update, :autocomplete]
+  before_action :load_projects, except: [:index, :new, :create, :projects, :edit, :update, :autocomplete]
   before_action :event_filter, only: :show
 
   layout :determine_layout

spec/controllers/groups_controller_spec.rb

@@ -0,0 +1,23 @@
+require 'rails_helper'
+
+describe GroupsController do
+  describe 'GET index' do
+    context 'as a user' do
+      it 'redirects to Groups Dashboard' do
+        sign_in(create(:user))
+
+        get :index
+
+        expect(response).to redirect_to(dashboard_groups_path)
+      end
+    end
+
+    context 'as a guest' do
+      it 'redirects to Explore Groups' do
+        get :index
+
+        expect(response).to redirect_to(explore_groups_path)
+      end
+    end
+  end
+end