Merge branch '18755-fix-destroy-project-causes-post_decline_request-to-be-executed' into 'master'

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, #18750.

## Does this MR meet the acceptance criteria?

- [x] No CHANGELOG needed.
- [x] Tests
  - [x] Added for this feature/bug
  - [x] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !4744

1 files changed, 2 insertions, 0 deletions

diff --git a/lib/gitlab/access.rb b/lib/gitlab/access.rb
index 6d0e30e916f..831f1e635ba 100644
--- a/lib/gitlab/access.rb
+++ b/lib/gitlab/access.rb
@@ -5,6 +5,8 @@
 #
 module Gitlab
   module Access
+    class AccessDeniedError < StandardError; end
+
     GUEST     = 10
     REPORTER  = 20
     DEVELOPER = 30