Get rid of user activites table and replace it with redis

• Gets rid of the user activities table
• Switch from postgreSQL to Redis

Fixes https://gitlab.com/gitlab-org/gitlab-ee/issues/1158

app/models/user.rb

@@ -96,7 +96,6 @@ class User < ActiveRecord::Base
   has_many :notification_settings,    dependent: :destroy
   has_many :award_emoji,              dependent: :destroy
   has_many :path_locks,               dependent: :destroy
-  has_one :user_activity, dependent: :destroy
 
   # Protected Branch Access
   has_many :protected_branch_merge_access_levels, dependent: :destroy, class_name: ProtectedBranch::MergeAccessLevel
@@ -153,7 +152,6 @@ class User < ActiveRecord::Base
   alias_attribute :private_token, :authentication_token
 
   delegate :path, to: :namespace, allow_nil: true, prefix: true
-  delegate :last_activity_at, to: :user_activity, allow_nil: true
 
   state_machine :state, initial: :active do
     event :block do
@@ -964,6 +962,12 @@ class User < ActiveRecord::Base
     end
   end
 
+  def record_activity
+    Gitlab::Redis.with do |redis|
+      redis.zadd('user/activities', Time.now.to_i, self.username)
+    end
+  end
+
   private
 
   # Returns a union query of projects that the user is authorized to access