Fix Error 500 in API when accessing a group that has an avatar

Closes #2340

CHANGELOG

 Please view this file on the master branch, on stable branches it's out of date.
 
 v 8.0.0 (unreleased)
+  - Fix Error 500 in API when accessing a group that has an avatar (Stan Hu)
   - Remove user OAuth tokens from the database and request new tokens each session (Stan Hu)
   - Only show recent push event if the branch still exists or a recent merge request has not been created (Stan Hu)
   - Remove satellites

app/models/group.rb

@@ -17,6 +17,7 @@ require 'carrierwave/orm/activerecord'
 require 'file_size_validator'
 
 class Group < Namespace
+  include Gitlab::ConfigHelper
   include Referable
 
   has_many :group_members, dependent: :destroy, as: :source, class_name: 'GroupMember'

spec/requests/api/groups_spec.rb

@@ -7,7 +7,8 @@ describe API::API, api: true  do
   let(:user2) { create(:user) }
   let(:user3) { create(:user) }
   let(:admin) { create(:admin) }
-  let!(:group1) { create(:group) }
+  let(:avatar_file_path) { File.join(Rails.root, 'spec', 'fixtures', 'banana_sample.gif') }
+  let!(:group1) { create(:group, avatar: File.open(avatar_file_path)) }
   let!(:group2) { create(:group) }
 
   before do