TF-4356 Add filter event mail in search (#4357)

* Add `Events` filter checkbox in Advanced Search form
* Perform search email by `Events` filter in Advanced Search form
* Add `Event` quick search button in Search view
* Use `keywords` replace to `headers` for search event filter

model/lib/extensions/keyword_identifier_extension.dart

@@ -6,6 +6,7 @@ import 'package:model/email/read_actions.dart';
 extension KeyWordIdentifierExtension on KeyWordIdentifier {
   static final unsubscribeMail = KeyWordIdentifier('\$unsubscribe');
   static final needsActionMail = KeyWordIdentifier('needs-action');
+  static final eventsMail = KeyWordIdentifier('event');
 
   String generatePath() => '${PatchObject.keywordsProperty}/$value';