Similar to https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/31834,
we see that in https://gitlab.com/gitlab-org/gitlab-ce/issues/65957
there can be hundreds, even thousands, of Gitaly requests in the
`/api/:version/projects/:id/merge_requests/:noteable_id/notes` endpoint.

Previously, the API to retrieve notes generated hundreds of Gitaly calls
to determine whether a system note should be shown to the user. It did
this by:

1. Rendering the Markdown
2. Extracting cross-references from the Markdown
3. Issuing a Gitaly `FindCommit` RPC for every reference to validate
that the commit exists.

The last step is unnecessary because we don't need to display a commit
if the user doesn't have access to the project in the first place.
`RendersNotes#prepare_notes_for_rendering` is already used in
`MergeRequestsController`, which is why we don't see N+1 Gitaly calls
there. We use it here to optimize the note redaction process.

Fix missing iid query on NotesFinder

Changed parameters of find_noteable, 
so changes across a few files were needed.
MergeRequest also requires iid instead of id query

Make NotesFinder fail with RecordNotFound again

Add specs for target_iid

Using RSpec tablesyntax for target_iid specs

Revert "Using RSpec tablesyntax for target_iid specs"

This reverts commit ba45c7f569a.

Allow find_by! here

Fix variable name

Add readable check

Revert "Add readable check"

This reverts commit 9e3a1a7aa39.

Remove unnecessary assignment

Add required changes for EE

Fix parameter count

Reduce code duplication by extracting a noteable module method

The call to find_noteable was redundant so
multiple files and lines have changed in that
commit to use the newly introduced module
method `noteable`.

Replace casecmp with include check

Add parent_type parameter


Revert "Reduce code duplication by extracting
a noteable module method"

This reverts commit 8c0923babff16.

Method is no longer needed

Check whether noteable can be read by user

This replaces API::Notes::NOTEABLE_TYPES and
API::Discussions::NOTEABLE_TYPES with class methods defined in a
separate helper modules. This allows EE to extend the list of noteable
types, without having to modify the constant in-place. We can't define
these methods directly in the API classes, as they would be used before
we're able to extend them in EE.

It also renames the API::PROJECT_ENDPOINT_REQUIREMENTS constant to
API::NAMESPACE_OR_PROJECT_REQUIREMENTS

Partially addresses #47424.

Had to make changes to spec files because
stubbing methods on frozen objects is a mess
in RSpec and leads to failures:

https://github.com/rspec/rspec-mocks/issues/1190

This whitelists all existing offenses for the various CodeReuse cops, of
which most are triggered by the CodeReuse/ActiveRecord cop.

* adds basic discussions API for issues and snippets
* reorganizes notes specs (so same tests can be used for all noteable types - issues, MRs, snippets)