added search intervention. search opens new tab with search in the java course (at least in chrome)

send only 3 interventions per exercise at maximum

db/migrate/20170228165741_add_search.rb

@@ -0,0 +1,10 @@
+class AddSearch < ActiveRecord::Migration
+  def change
+    create_table :searches do |t|
+      t.belongs_to :exercise, null: false
+      t.belongs_to :user, polymorphic: true, null: false
+      t.string :search
+      t.timestamps
+    end
+  end
+end