Add confidential issues

Closes gitlab-org/gitlab-ce#3678

Tasks:

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

Screenshots:

* New issue (1):

![new-issue-1](/uploads/b07f8e72cb2183492c142fdeba7ad8a1/new-issue-1.png)

* New issue (2):

![new-issue-2](/uploads/ac3d6ed96d1e5ab95b076bc09c829b3e/new-issue-2.png)

* Issues:

![issues](/uploads/2c891fbe536962a1501723b4cb4681a3/issues.png)

* Issue:

![issue](/uploads/5b11db32ea618c590fc378f21589dd0c/issue.png)

* Search:

![search](/uploads/88591dfc794d7bce097c72d286c87610/search.png)

* Milestone:

![milestone](/uploads/c26b48a1786514b3a3d66026053e9277/milestone.png)

See merge request !227

ES: Index recreation



See merge request !264

Handle duplicate appearances table creation issue with upgrade from CE to EE

Closes #391

See merge request !278

Hide builds from Project's settings when the feature is disabled (EE)

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/13944

See merge request !268

Geo: ReadOnly middleware improvements

We should handle API requests (JSON) in a different way we handle normal requests.

Current implementation always redirect to root and display error messages using `flash[:alert]`.
This MR adds `{ message: "..." }` type of response when request is under `application/json` content_type.

This is a small patch/enhancement for Geo (#76)

See merge request !274

Closes #391

Ensure GithubImport::Client does not crash when the 'verify_ssl' config is not present

This fixes gitlab-org/gitlab-ce#14343 and !260 when `verify_ssl` is not present in the GitHub omniauth
settings in `gitlab.yml`.

Settingslogic allow to access optional settings through the Hash
notation: if the key is not present, it returns nil. In the present case
this will let the Octokit::Client object use whatever default value it
wants for the ssl option.

/cc @dbalexandre 

See merge request !275

Settingslogic allow to access optional settings through the Hash
notation: if the key is not present, it returns nil. In the present case
this will let the Octokit::Client object use whatever default value it
wants for the ssl option.

Fix Elastic::Indexer spec

This break was broken by !273.

See merge request !277