Fix duplicate invites (#1437)

Co-authored-by: Josh Pigford <josh@joshpigford.com>

app/models/invitation.rb

@@ -5,6 +5,7 @@ class Invitation < ApplicationRecord
   validates :email, presence: true, format: { with: URI::MailTo::EMAIL_REGEXP }
   validates :role, presence: true, inclusion: { in: %w[admin member] }
   validates :token, presence: true, uniqueness: true
+  validates_uniqueness_of :email, scope: :family_id, message: "has already been invited to this family"
   validate :inviter_is_admin
 
   before_validation :generate_token, on: :create