Allow import/export of mutes list (#1541)

* Allow export of mutes list

* Allow importing of mutes list

* Refactor to use Settings::Exports::BaseController and DRY up exports code

app/controllers/settings/exports/muted_accounts_controller.rb

@@ -0,0 +1,13 @@
+# frozen_string_literal: true
+
+module Settings
+  module Exports
+    class MutedAccountsController < BaseController
+      private
+
+      def export_accounts
+        current_account.muting
+      end
+    end
+  end
+end