Create ActiveRecordModel and table for Merge Train feature

This database table and AR model is going to be used for
the merge train feature.

db/migrate/20190418182545_create_merge_request_trains_table.rb

+# frozen_string_literal: true
+
+class CreateMergeRequestTrainsTable < ActiveRecord::Migration[5.1]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  def change
+    create_table :merge_trains, id: :bigserial do |t|
+      t.references :merge_request, foreign_key: { on_delete: :cascade }, type: :integer, index: false, null: false
+      t.references :user, foreign_key: { on_delete: :cascade }, type: :integer, null: false
+      t.references :pipeline, foreign_key: { to_table: :ci_pipelines, on_delete: :nullify }, type: :integer
+      t.timestamps_with_timezone null: false
+
+      t.index [:merge_request_id], unique: true
+    end
+  end
+end

db/schema.rb

@@ -1361,6 +1361,17 @@ ActiveRecord::Schema.define(version: 20190426180107) do
     t.index ["merge_request_id"], name: "index_merge_requests_closing_issues_on_merge_request_id", using: :btree
   end
 
+  create_table "merge_trains", force: :cascade do |t|
+    t.integer "merge_request_id", null: false
+    t.integer "user_id", null: false
+    t.integer "pipeline_id"
+    t.datetime_with_timezone "created_at", null: false
+    t.datetime_with_timezone "updated_at", null: false
+    t.index ["merge_request_id"], name: "index_merge_trains_on_merge_request_id", unique: true, using: :btree
+    t.index ["pipeline_id"], name: "index_merge_trains_on_pipeline_id", using: :btree
+    t.index ["user_id"], name: "index_merge_trains_on_user_id", using: :btree
+  end
+
   create_table "milestones", id: :serial, force: :cascade do |t|
     t.string "title", null: false
     t.integer "project_id"
@@ -2514,6 +2525,9 @@ ActiveRecord::Schema.define(version: 20190426180107) do
   add_foreign_key "merge_requests", "users", column: "updated_by_id", name: "fk_641731faff", on_delete: :nullify
   add_foreign_key "merge_requests_closing_issues", "issues", on_delete: :cascade
   add_foreign_key "merge_requests_closing_issues", "merge_requests", on_delete: :cascade
+  add_foreign_key "merge_trains", "ci_pipelines", column: "pipeline_id", on_delete: :nullify
+  add_foreign_key "merge_trains", "merge_requests", on_delete: :cascade
+  add_foreign_key "merge_trains", "users", on_delete: :cascade
   add_foreign_key "milestones", "namespaces", column: "group_id", name: "fk_95650a40d4", on_delete: :cascade
   add_foreign_key "milestones", "projects", name: "fk_9bd0a0c791", on_delete: :cascade
   add_foreign_key "note_diff_files", "notes", column: "diff_note_id", on_delete: :cascade