Fix subsequent SAML sign ins

Fixes a bug when `auto_link_ldap_user` is `true` that causes SAML users to be unable to sign in a second time.

Fix the problem for https://gitlab.zendesk.com/agent/tickets/22546

See merge request !4718

Handle external issues in IssueReferenceFilter

Handling of external issues was broken when I refactored
`IssueReferenceFilter` to use fewer SQL queries.

Fixes #18827

See merge request !4789

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

Move pre_process into render_result

This MR moves `Banzai::Renderer.pre_process` into `Banzai::Renderer.render_result`.

The `pre_process` method was called even when its output would be ignored. See 11a5a4f359ee57029dbfcc9185fc6b47243ea2aa for more details.

See merge request !4830

In the past this class would use Project#get_issue to retrieve an issue
by its ID. This method would automatically determine whether to return
an Issue or ExternalIssue.

This commit changes IssueReferenceFilter to handle external issues again
and in a somewhat more explicit manner than before.

Fixes gitlab-org/gitlab-ce#18827