Add basic invitations

db/migrate/20201130132533_create_invitations.rb

@@ -0,0 +1,12 @@
+class CreateInvitations < ActiveRecord::Migration[6.0]
+  def change
+    create_table :invitations do |t|
+      t.string :token
+      t.integer :user_id
+      t.integer :invited_user_id
+      t.datetime :used_at
+
+      t.timestamps
+    end
+  end
+end