Replace MR access checks with use of MergeRequestsFinder

Split from !2024 to partially solve https://gitlab.com/gitlab-org/gitlab-ce/issues/23867

⚠ - Potentially untested
💣 - No test coverage
🚥 - Test coverage of some sort exists (a test failed when error raised)
🚦 - Test coverage of return value (a test failed when nil used)
✅ - Permissions check tested

- [x] 💣  app/finders/notes_finder.rb:17
- [x] ⚠  app/views/layouts/nav/_project.html.haml:80 [`.count`]
- [x] 💣  app/controllers/concerns/creates_commit.rb:84
- [x] 🚥  app/controllers/projects/commits_controller.rb:24
- [x] 🚥  app/controllers/projects/compare_controller.rb:56
- [x] 🚦  app/controllers/projects/discussions_controller.rb:29
- [x] ✅  app/controllers/projects/todos_controller.rb:27
- [x] 🚦  app/models/commit.rb:268
- [x] ✅ lib/gitlab/search_results.rb:71

- [x] https://dev.gitlab.org/gitlab/gitlabhq/merge_requests/2024/diffs#d1c10892daedb4d4dd3d4b12b6d071091eea83df_267_266 Memoize ` merged_merge_request(current_user)`
- [x] https://dev.gitlab.org/gitlab/gitlabhq/merge_requests/2024/diffs#d1c10892daedb4d4dd3d4b12b6d071091eea83df_248_247 Expected side effect for `merged_merge_request!`, consider `skip_authorization: true`.
- [x] https://dev.gitlab.org/gitlab/gitlabhq/merge_requests/2024/diffs#d1c10892daedb4d4dd3d4b12b6d071091eea83df_269_269 Scary use  of unchecked `merged_merge_request?`

See merge request !2033