initialize invites

Invites model,controller and views are initialized.

add datepicker and selectize in invites#new

calender is added for end date field and selectize is added for emails field to allow multiple emails in field

db/migrate/20190810082925_create_invites.rb

@@ -0,0 +1,12 @@
+class CreateInvites < ActiveRecord::Migration[5.2]
+  def change
+    create_table :invites do |t|
+      t.integer :conference_id
+      t.integer :user_id
+      t.date :end_date
+      t.text :invite_for
+
+      t.timestamps
+    end
+  end
+end