Replace issue access checks with use of IssuableFinder
Split from !2024 to partially solve https://gitlab.com/gitlab-org/gitlab-ce/issues/23867
## Which fixes are in this MR?
:warning: - Potentially untested
:bomb: - No test coverage
:traffic_light: - Test coverage of some sort exists (a test failed when error raised)
:vertical_traffic_light: - Test coverage of return value (a test failed when nil used)
:white_check_mark: - Permissions check tested
### Issue lookup with access check
Using `visible_to_user` likely makes these security issues too. See [Code smells](#code-smells).
- [x] :vertical_traffic_light: app/finders/notes_finder.rb:15 [`visible_to_user`]
- [x] :traffic_light: app/views/layouts/nav/_project.html.haml:73 [`visible_to_user`] [`.count`]
- [x] :white_check_mark: app/services/merge_requests/build_service.rb:84 [`issue.try(:confidential?)`]
- [x] :white_check_mark: lib/api/issues.rb:112 [`visible_to_user`]
- CHANGELOG: Prevented API returning issues set to 'Only team members' to everyone
- [x] :white_check_mark: lib/api/helpers.rb:126 [`can?(current_user, :read_issue, issue)`] Maybe here too?
- [x] :white_check_mark: lib/gitlab/search_results.rb:53 [`visible_to_user`]
### Previous discussions
- [ ] https://dev.gitlab.org/gitlab/gitlabhq/merge_requests/2024/diffs#b2ff264eddf9819d7693c14ae213d941494fe2b3_128_126
- [ ] https://dev.gitlab.org/gitlab/gitlabhq/merge_requests/2024/diffs#7b6375270d22f880bdcb085e47b519b426a5c6c7_87_87
See merge request !2031