add configurable email subject suffix

What does this MR do?

add a configurable email subject suffix

Are there points in the code the reviewer needs to double check?

not too many differences

Why was this MR needed?

I noticed that there is a commit 44ab156e 3 years ago which removes email subject prefix

actually we do need an special part of the email subject which makes the email subject easy to identify

we just need to make it configurable rather than remove it.

What are the relevant issue numbers?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together