Add possibility to not send emails for users that have emailstop=1

[mattiabonzi]

This update introduces a new feature to the mod_reengagement plugin, allowing the exclusion of users who have the "emailstop" preference set to true from receiving emails.

• The functionality is controlled by a flag at the activity level.
• By default, the flag is set to false to preserve the existing behaviour, ensuring emails are sent to all users as before.

[danmarsden]

Hi @mattiabonzi this feels a little strange to be implemented within the plugin itself - the plugin uses moodle's core api "message_send" which already checks emalstop.

can you provide a bit more information about how users are getting emails when they have emailstop enabled? - the core api should be preventing it.

Looking through the code I do wonder if the "thirdpartyemails" setting is set and it's those users who are getting emails despite the fact they have emailstop enabled? - in which case the patch you've added won't help either.