Replace issue access checks with use of IssuableFinder

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

Using `visible_to_user` likely makes these security issues too. See [Code smells](#code-smells).

- [x]  app/finders/notes_finder.rb:15 [`visible_to_user`]
- [x]  app/views/layouts/nav/_project.html.haml:73 [`visible_to_user`] [`.count`]
- [x]  app/services/merge_requests/build_service.rb:84 [`issue.try(:confidential?)`]
- [x]  lib/api/issues.rb:112 [`visible_to_user`]
  - CHANGELOG: Prevented API returning issues set to 'Only team members' to everyone
- [x]  lib/api/helpers.rb:126 [`can?(current_user, :read_issue, issue)`] Maybe here too?
- [x]  lib/gitlab/search_results.rb:53 [`visible_to_user`]

- [ ] 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
Signed-off-by: Rémy Coutable <remy@rymai.me>

There's no need to order queries used as sub-queries and doing so can
add potential overhead.

Instead of plucking IDs this class now uses ActiveRecord::Relation
objects. Plucking IDs is problematic as searching for projects can lead
to a huge amount of IDs being loaded into memory only to be used as an
argument for another query (instead of just using a sub-query).

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Gitlab::SearchResults and Gitlab::ProjectSearchResults are libraries we
are going to use to get search results based on query, enitity type and
pagination.

It will allow us to get only issues from project #23 where title or
description includes 'foo'.

Ex:

search_results = Gitlab::ProjectSearchResults.new(project.id, 'foo', 'issues')

search_results.objects => # [<Issues #23>, <Issues #34>]
search_results.issues_count => 2
search_results.total_count => 12 (it includes results from comments and
merge requests too)
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>