Fix not being able to change world filter expiration back to “Never” (#15858)

Fixes #15849

app/models/concerns/expireable.rb

@@ -17,7 +17,7 @@ module Expireable
     end
 
     def expires_in=(interval)
-      self.expires_at = interval.to_i.seconds.from_now if interval.present?
+      self.expires_at = interval.present? ? interval.to_i.seconds.from_now : nil 
       @expires_in     = interval
     end
 

app/views/filters/_fields.html.haml

@@ -2,7 +2,7 @@
   .fields-row__column.fields-row__column-6.fields-group
     = f.input :phrase, as: :string, wrapper: :with_label, hint: false
   .fields-row__column.fields-row__column-6.fields-group
-    = f.input :expires_in, wrapper: :with_label, collection: [30.minutes, 1.hour, 6.hours, 12.hours, 1.day, 1.week].map(&:to_i), label_method: lambda { |i| I18n.t("invites.expires_in.#{i}") }, prompt: I18n.t('invites.expires_in_prompt')
+    = f.input :expires_in, wrapper: :with_label, collection: [30.minutes, 1.hour, 6.hours, 12.hours, 1.day, 1.week].map(&:to_i), label_method: lambda { |i| I18n.t("invites.expires_in.#{i}") }, include_blank: I18n.t('invites.expires_in_prompt')
 
 .fields-group
   = f.input :context, wrapper: :with_block_label, collection: CustomFilter::VALID_CONTEXTS, as: :check_boxes, collection_wrapper_tag: 'ul', item_wrapper_tag: 'li', label_method: lambda { |context| I18n.t("filters.contexts.#{context}") }, include_blank: false