PROD-948 Fix - Group message email notification is not sending when the organizer has an Instructor role

[kent-carlo]

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Changes proposed in this Pull Request:

Fixes #937

How to test the changes in this Pull Request:

1. Make sure that you have BuddyBoss Platform, LearnDash LMS, and Instructor Role plugins installed and activated.
2. Enable BuddyBoss > Settings > Groups > Group Messages.
3. Create a user with Instructor Role.
4. In the BuddyBoss platform, create a Social Group and set Instructor user as an Organizer and add some members.
5. Log in as an Instructor user and try to send a group message. Groups > Send Messages

Proof Screenshots or Video

Other information:

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• [x] Have you written new tests for your changes, as applicable?
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Changelog entry

Added exclusion in bp-email post type as it causes for email not to work if sender has instructor role from instructor-role plugin