Improve task for generating invitations

Turn the argument into the target number of unused invitations for each
user, thus not generating more invitations for users who already have a
sufficient amount of unused ones.

lib/tasks/invitations.rake

@@ -2,12 +2,21 @@ namespace :invitations do
   desc "Generate invitations for all users"
   task :generate_for_all_users, [:amount_per_user] => :environment do |t, args|
     count = 0
+
     User.all.each do |user|
-      args[:amount_per_user].to_i.times do
+      amt_to_create = args[:amount_per_user].to_i - user.invitations.unused.count
+      next unless amt_to_create > 0
+      amt_created = 0
+
+      amt_to_create.times do
         user.invitations << Invitation.create(user: user)
         count += 1
+        amt_created += 1
       end
+
+      puts "Created #{amt_created} new invitations for #{user.address}"
     end
-    puts "Created #{count} new invitations"
+
+    puts "---\nCreated #{count} new invitations overall"
   end
 end