Add post-migration to populate user_contributed_projects table.

Trying to be a good citizen here, we spread the load building that
table. Maybe not required, but at least good practice and no harm.

db/post_migrate/20180223124427_build_user_contributed_projects_table.rb

+class BuildUserContributedProjectsTable < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  # Set this constant to true if this migration requires downtime.
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  BATCH_SIZE = 100000
+  SLEEP_TIME = 30
+
+  def up
+    with_index(:events, [:author_id, :project_id], name: 'events_user_contributions_temp', where: 'project_id IS NOT NULL') do
+      iteration = 0
+      records = 0
+      begin
+        Rails.logger.info "Building user_contributed_projects table, batch ##{iteration}"
+        result = execute <<~SQL
+          INSERT INTO user_contributed_projects (user_id, project_id)
+          SELECT e.user_id, e.project_id
+          FROM (SELECT DISTINCT author_id AS user_id, project_id FROM events WHERE project_id IS NOT NULL) AS e
+          LEFT JOIN user_contributed_projects ucp USING (user_id, project_id)
+          WHERE ucp.user_id IS NULL
+          LIMIT #{BATCH_SIZE}
+        SQL
+        iteration += 1
+        records += result.cmd_tuples
+        Rails.logger.info "Building user_contributed_projects table, batch ##{iteration} complete, created #{records} overall"
+        Kernel.sleep(SLEEP_TIME) if result.cmd_tuples > 0
+      end while result.cmd_tuples > 0
+    end
+
+    execute "ANALYZE user_contributed_projects" if Gitlab::Database.postgresql?
+  end
+
+  def down
+    execute "TRUNCATE user_contributed_projects"
+  end
+
+  private
+
+  def with_index(*args)
+    add_concurrent_index(*args) unless index_exists?(*args)
+    yield
+  ensure
+    remove_concurrent_index(*args) if index_exists?(*args)
+  end
+end