Thread scopes through #matches?. #454.

Also add an apply_to_mentions attribute on Glitch::KeywordMute, which is
used to calculate scope.  Next up: additions to the test suite to
demonstrate how scoping works.

db/migrate/20180604000556_add_apply_to_mentions_flag_to_keyword_mutes.rb

@@ -0,0 +1,17 @@
+require 'mastodon/migration_helpers'
+
+class AddApplyToMentionsFlagToKeywordMutes < ActiveRecord::Migration[5.2]
+  include Mastodon::MigrationHelpers
+
+  disable_ddl_transaction!
+
+  def up
+    safety_assured do
+      add_column_with_default :glitch_keyword_mutes, :apply_to_mentions, :boolean, allow_null: false, default: true
+    end
+  end
+
+  def down
+    remove_column :glitch_keyword_mutes, :apply_to_mentions
+  end
+end