Dynamic content in the notification by email

[mckillo]

From forum: Modifying Emails sent to registered attendees without venue
JEM does not have any option to customize the fields in email notifications.

Now, the notifications are static, based on the content of jem.ini, such as PLG_JEM_MAILER_USER_REG_ON_WAITING_BODY_9, and the %s it contains, with a fixed order in the sprintf of the code.

The proposal consists of two phases:

1. Replace all %s with placeholders like {event_title}, {event_description}, {attendeename}, etc. This will make the content dynamic. The admin should be able to define the text template for each notification email, change the order in which they want to place the fields, and decide which fields to include.

2. Store the custom notifications in the database. This is more complex in the code; it represents a major update to the mailer plugin and poses a challenge for multilingual support.

We need to rethink this before proceeding with further discussion.