Clarify use of email felds for source and destination user accounts

**Describe the bug**
The `email` entity only has two entries, and does not define how email sender and recipient are to be defined; these are defined in the "user" field - `user_email`, `source_user_email`, `destination_user_email`.

Review if this shared use of the `email` and `user` entities is appropriate, or if the email fields should depend on context:
- User properties metadata using the user_... object to add email information - `user_email`, `source_user_email`, `destination_user_email`
- Email message properties using email_ fields `email_sender`, `email_recipient` (or similar)
- With an `associated_email` copy_to destination added to the template

**Graylog Version (please complete the following information):**
- Graylog Version: 4.2.7
- Illuminate Version: 2.2.0 RC2