Properly save suggestions in project exports

Previously imports would fail if a merge request note included a
suggestion with an
`ActiveRecord::HasManyThroughCantAssociateThroughHasOneOrManyReflection`
exception.

This was happening because suggestions were listed as a descendant of
merge requests, but this doesn't work because suggestions are directly
associated with notes, not merge requests, and that association is lost.
Rails also disallows creating intializing a has_many association through
a different object.

We fix this by making `suggestions` a child of `notes` within a merge
request. This doesn't fix previously broken exported project exports,
but new exports will work.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/65880

changelogs/unreleased/sh-fix-import-export-suggestions.yml

+---
+title: Properly save suggestions in project exports
+merge_request: 31690
+author:
+type: fixed

lib/gitlab/import_export/import_export.yml

@@ -37,11 +37,11 @@ project_tree:
     - :user
   - merge_requests:
     - :metrics
-    - :suggestions
     - notes:
       - :author
       - events:
         - :push_event_payload
+      - :suggestions
     - merge_request_diff:
       - :merge_request_diff_commits
       - :merge_request_diff_files

spec/lib/gitlab/import_export/project.json

@@ -2450,7 +2450,21 @@
           "author": {
             "name": "Ottis Schuster II"
           },
-          "events": []
+          "events": [],
+          "suggestions": [
+            {
+              "id": 1,
+              "note_id": 674,
+              "relative_order": 0,
+              "applied": false,
+              "commit_id": null,
+              "from_content": "Original line\n",
+              "to_content": "New line\n",
+              "lines_above": 0,
+              "lines_below": 0,
+              "outdated": false
+            }
+          ]
         },
         {
           "id": 675,

spec/lib/gitlab/import_export/project_tree_restorer_spec.rb

@@ -125,6 +125,13 @@ describe Gitlab::ImportExport::ProjectTreeRestorer do
         expect(MergeRequest.find_by(title: 'MR1').resource_label_events).not_to be_empty
       end
 
+      it 'restores suggestion' do
+        note = Note.find_by("note LIKE 'Saepe asperiores exercitationem non dignissimos laborum reiciendis et ipsum%'")
+
+        expect(note.suggestions.count).to eq(1)
+        expect(note.suggestions.first.from_content).to eq("Original line\n")
+      end
+
       context 'event at forth level of the tree' do
         let(:event) { Event.where(action: 6).first }