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>
Implement custom notification level options


part of #12758
See merge request !4389
+ Move 'Edit Project/Group' out of membership-related partial
+ Show the access request buttons only to logged-in users
+ Put the request access buttons out of in a more visible button
+ Improve the copy in the #remove_member_message helper
Signed-off-by: Rémy Coutable <remy@rymai.me>
Two issues:
1. The constraints in the resources were incorrect. Here's what it was before:
```
group_milestone GET /groups/:group_id/milestones/:id(.:format) groups/milestones#show {:id=>/[a-zA-Z.0-9_\-]+(?<!\.atom)/, :group_id=>/[a-zA-Z.0-9_\-]+(?<!\.atom)/}
```
In this case, id is actually the title of the milestone, which can be anything at the moment.
After:
```
group_milestone GET /groups/:group_id/milestones/:id(.:format) groups/milestones#show {:id=>/[^\/]+/, :group_id=>/[a-zA-Z.0-9_\-]+(?<!\.atom)/}
```
2. `parameterize` would strip all Unicode characters, leaving a blank string. Rails would report something like:
ActionView::Template::Error (No route matches {:action=>"show", :controller=>"groups/milestones", :group_id=>#<Group id: 48, name: "ops-dev", path: "ops-dev", owner_id: nil, created_at: "2015-11-15 08:55:30", updated_at: "2015-12-02 06:23:26", type: "Group", description: "", avatar: "sha1.c71e73d51af1865c1bbbf6208e10044d46c9bb93.png", public: false>, :id=>"", :title=>"肯定不是中文的问题"} missing required keys: [:id]):
This change uses the babosa library to create a better slug, which surprisingly
isn't actually used by the global milestone controllers. Instead, they use the
title passed as a query string for some reason.
Closes https://github.com/gitlabhq/gitlabhq/issues/9881
Fix constraints
This would result in a 404 error in certain views.
The `save` call was being rolled back due to an error in the validation step.
Relax the validation step so that this works.
Closes#1570