add unsubscribe to email

If a user has opted out of emails then invitation mail will not be sent to the user

app/controllers/concerns/invitation.rb

@@ -7,7 +7,7 @@ module Invitation
 
       emails_array.each do |email|
         new_user = User.find_by(email: email)
-        if new_user.nil?
+        if new_user.nil? || !new_user.opted_out?
           User.invite!({ email: email }, current_user)
           new_user = User.find_by(email: email)
         end

app/views/devise/mailer/invitation_instructions.html.erb

@@ -9,3 +9,5 @@
 <% end %>
 
 <p><%= t("devise.mailer.invitation_instructions.ignore") %></p>
+<hr>
+<small>Click on <%= link_to "Unsubscribe", mailkick_unsubscribe_url %>, to stop receiving invitation mails.</small>

app/views/devise/mailer/invitation_instructions.text.erb

@@ -9,3 +9,5 @@
 <% end %>
 
 <%= t("devise.mailer.invitation_instructions.ignore") %>
+
+Click on <%= mailkick_unsubscribe_url %>, to stop receiving invitation mails.