Use entity title in confirmation email body and in subject

Problem/Motivation

The confirmation email currently uses entity_value in both the
email subject and the email body when referring to the page being followed.
This displays the entity value instead of the human-readable page title.

The confirmation email should use entity_title for user-facing
references to the page.

The email body also contains two minor wording issues: “Thank you for
interest” should be “Thank you for your interest”.

Steps to reproduce

1. Configure the Symfony Mailer integration for the
   de_notifications module.
2. Create or use a notification subscription for a page.
3. Trigger the confirmation email.
4. Inspect the email subject and body.

Both the subject and the body use entity_value, resulting in the
technical entity value being displayed instead of the human-readable page
title.

Proposed resolution

Update the confirmation email configuration in
de_notifications_symfony_mailer/config/install/symfony_mailer.mailer_policy.de_notifications_mailer.confirm.yml.

Replace {{ entity_value }} with
{{ entity_title }} in both:

• the confirmation email subject;
• the confirmation email body.

Also correct the wording in the email body:

• Change “Thank you for interest” to “Thank you for your interest”.

Remaining tasks

• Confirm that entity_title is available in the confirmation
  email context.
• Update the subject to use entity_title.
• Update the body to use entity_title.
• Test the generated confirmation email.
• Confirm that the page title is displayed correctly in both the subject
  and the body.
• Confirm that the confirmation URL still works as expected.

User interface changes

The confirmation email subject and body will display the human-readable page
title instead of the technical entity value. The wording in the email body
will also be corrected.

API changes

None.

Data model changes

None.