feature(site_notifications): option to mass delete site notifications …

When a user has a lot of site notifications he/she has to delete them
individually. This is very time consuming, so this solves that problem

fixes #6219
i copied it from the great messages plugin
https://github.com/Elgg/Elgg/blob/1.x/mod/messages/views/default/forms/messages/process.php#L38
https://github.com/Elgg/Elgg/blob/1.x/mod/messages/languages/en.php#L25