Clarify use of email felds for source and destination user accounts

[miwent]

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