Adding exclusive lists feature

Major new feature, currently testing in a branch right now. Fixes #1

app/models/list.rb

@@ -3,11 +3,12 @@
 #
 # Table name: lists
 #
-#  id         :bigint(8)        not null, primary key
-#  account_id :bigint(8)        not null
-#  title      :string           default(""), not null
-#  created_at :datetime         not null
-#  updated_at :datetime         not null
+#  id           :bigint(8)        not null, primary key
+#  account_id   :bigint(8)        not null
+#  title        :string           default(""), not null
+#  created_at   :datetime         not null
+#  updated_at   :datetime         not null
+#  is_exclusive :boolean          default(FALSE)
 #
 
 class List < ApplicationRecord