Add Member AR model

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/models/member.rb

+class Member < ActiveRecord::Base
+  include Notifiable
+  include Gitlab::Access
+
+  belongs_to :user
+  belongs_to :source, polymorphic: true
+
+  validates :user, presence: true
+  validates :source, presence: true
+  validates :user_id, uniqueness: { scope: [:source_type, :source_id], message: "already exists in source" }
+  validates :access_level, inclusion: { in: Gitlab::Access.values }, presence: true
+
+  scope :guests, -> { where(group_access: GUEST) }
+  scope :reporters, -> { where(group_access: REPORTER) }
+  scope :developers, -> { where(group_access: DEVELOPER) }
+  scope :masters,  -> { where(group_access: MASTER) }
+  scope :owners,  -> { where(group_access: OWNER) }
+end

spec/models/members_spec.rb

+require 'spec_helper'
+
+describe Member do
+  describe "Associations" do
+    it { should belong_to(:user) }
+  end
+
+  describe "Validation" do
+    subject { Member.new(access_level: Member::GUEST) }
+
+    it { should validate_presence_of(:user) }
+    it { should validate_presence_of(:source) }
+    it { should ensure_inclusion_of(:access_level).in_array(Gitlab::Access.values) }
+  end
+end