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
Signed-off-by: Rémy Coutable <remy@rymai.me>
Douwe MaanandRémy Coutable
Dmitriy Zaporozhets