Resolve "Destroying a project causes post_decline_request to be executed"

What does this MR do?

Ensure we don't send "access request declined" to access requesters when a project is deleted.

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

I've created a service to decouple the notification sending from the AR model.

Why was this MR needed?

Because there was an issue.

What are the relevant issue numbers?

Fixes #18755 (closed), #18750 (closed).

Does this MR meet the acceptance criteria?

• No CHANGELOG needed.
• 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