FIX: N+1 for admins viewing groups page

Groups page was loading fields that are only used on the group show
page, so move those fields to the GroupShowSerializer.
Also only fetch the default category and tag notifications once.

spec/serializers/basic_group_serializer_spec.rb

@@ -40,22 +40,6 @@ describe BasicGroupSerializer do
     end
   end
 
-  describe '#automatic_membership_email_domains' do
-    fab!(:group) { Fabricate(:group, automatic_membership_email_domains: 'ilovediscourse.com') }
-    let(:admin_guardian) { Guardian.new(Fabricate(:admin)) }
-
-    it 'should include email domains for admin' do
-      subject = described_class.new(group, scope: admin_guardian, root: false, owner_group_ids: [group.id])
-      expect(subject.as_json[:automatic_membership_email_domains]).to eq('ilovediscourse.com')
-    end
-
-    it 'should not include email domains for other users' do
-      subject = described_class.new(group, scope: guardian, root: false, owner_group_ids: [group.id])
-      expect(subject.as_json[:automatic_membership_email_domains]).to eq(nil)
-    end
-
-  end
-
   describe '#has_messages' do
     fab!(:group) { Fabricate(:group, has_messages: true) }
 
@@ -113,26 +97,4 @@ describe BasicGroupSerializer do
       end
     end
   end
-
-  describe 'admin only fields' do
-    fab!(:group) { Fabricate(:group, email_username: 'foo@bar.com', email_password: 'pa$$w0rd') }
-
-    describe 'for a user' do
-      let(:guardian) { Guardian.new(Fabricate(:user)) }
-
-      it 'are not visible' do
-        expect(subject.as_json[:email_username]).to be_nil
-        expect(subject.as_json[:email_password]).to be_nil
-      end
-    end
-
-    describe 'for an admin' do
-      let(:guardian) { Guardian.new(Fabricate(:admin)) }
-
-      it 'are visible' do
-        expect(subject.as_json[:email_username]).to eq('foo@bar.com')
-        expect(subject.as_json[:email_password]).to eq('pa$$w0rd')
-      end
-    end
-  end
 end