Email notifications

[amieiro]

This PR adds email notifications to all the attendees of an event around 1 hour and 1 day before the event.

To test this PR, I recommend you this configuration:

• WP Crontrol plugin to view the scheduled jobs.
• Mailtrap or some similar tool to be able to easily review the emails sent by this PR.
• WP Mail SMTP or another SMTP plugin to easily configure the email sending to Mailtrap.

To test this PR, you can:

Create an event. In this example, I created an event for the May month. As I am locally work in CEST timezone (UTC+2) and the form is using UTC, I need to start on 2024-04-30 22:00 (2024-05-01 00:00 UTC time) and finish on 2024-05-31 21:59 (2024-05-31 23:59 UTC time).

Once I published the event, with the WP Crontrol plugin I can see the custom cron events (in CEST timezone):

• One 1 hour before the event: 2024-04-30 23:00 CEST.
• One 23 hours before the event: 2024-04-30 00:00 CEST.

You can see in the notifications:

• The post_id (event_id) as parameter.
• The Wporg\TranslationEvents\Notifications\Notifications_Send->send_notifications() as the action that will be executed when this event will be fired.

Each time I modified the event, all these custom cron events are deleted and re-scheduled, to use the new start time.

To fire one of these events manually, you can use the Run now link, available in each custom cron event.

Once the custom cron event is fired, you will receive an email like this:

Fixes #168.

[akirk]

Added some ideas to improve the e-mail text.

[psrpinto]

Fantastic work @amieiro! I really like the separation into Notifications_Schedule and Notifications_Send, makes things very easy to understand.

[akirk]

Great, sorry for the false alarm of something not implemented. It looks good now! I thought we had said we'd wanted to show how many other people are also attending the event but we can do that in a follow-up PR. Good to get this one merged.