Add confidential issues

Closes gitlab-org/gitlab-ce#3678

Tasks:

• Add confidential flag to issues table
• Allow user to mark/unmark an issue as confidential
• Restrict access to confidential issues for non-members/author/assignee
  • Issues list
  • Issue details
  • API
  • Search results when Elasticsearch is disabled
  • Search results when Elasticsearch is enabled, and search by term
  • Search results when Elasticsearch is enabled, and search by iid
• Remove references for confidential issues
  • Issue/MR Description
  • Notes
  • Autocomplete
• Milestone overview
  • Hide confidential issues for non-members/author/assignee
  • Does not count confidential issues for non-members/author/assignee
• Add a lock icon to confidential issues
  • Issues list
  • Issue details
  • Search results
• Activity Feed
  • Hide confidential issues for non-members/author/assignee

Screenshots:

• New issue (1):

• New issue (2):

• Issues:

• Issue:

• Search:

• Milestone: