Add timeline read markers API (#11762)

Fix #4093

db/migrate/20190904222339_create_markers.rb

@@ -0,0 +1,14 @@
+class CreateMarkers < ActiveRecord::Migration[5.2]
+  def change
+    create_table :markers do |t|
+      t.references :user, foreign_key: { on_delete: :cascade, index: false }
+      t.string :timeline, default: '', null: false
+      t.bigint :last_read_id, default: 0, null: false
+      t.integer :lock_version, default: 0, null: false
+
+      t.timestamps
+    end
+
+    add_index :markers, [:user_id, :timeline], unique: true
+  end
+end