Fix expiration date of filters being set to “Never” when editing them (…

…mastodon#11204)

When editing a custom filter, select the shortest preset duration that
still covers the remaining time of that filter.

Fixes mastodon#9506

app/models/custom_filter.rb

@@ -35,6 +35,13 @@ class CustomFilter < ApplicationRecord
   before_validation :clean_up_contexts
   after_commit :remove_cache
 
+  def expires_in
+    return @expires_in if defined?(@expires_in)
+    return nil if expires_at.nil?
+
+    [30.minutes, 1.hour, 6.hours, 12.hours, 1.day, 1.week].find { |expires_in| expires_in.from_now >= expires_at }
+  end
+
   private
 
   def clean_up_contexts