Add option to "Trigger builds for mirror updates"

Fixes https://gitlab.com/gitlab-org/gitlab-ee/issues/190 and
https://gitlab.com/gitlab-org/gitlab-ce/issues/3647

See https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/125

Add minimum requirements for Elasticsearch



See merge request !130

[ci skip]

Elastic search improvements

* Remove warning when you run index_repositories task first time and there is no index yet.

* Update wiki index dynamically. It has been forgotten.


See merge request !126

Update changelog



See merge request !128

Elasticsearch integration

https://gitlab.com/gitlab-org/gitlab-ee/issues/61

Steps:

- [x] Base integration
- [x] Implement snippet search (service lawyer and actual search)
- [x] Implement project search (service lawyer and actual search)
- [x] Implement global search (service lawyer and actual search)
- [x] Implement search through the notes
- [x] Implement search through the milestones
- [x] Implement search though the wiki blobes
- [x] Implement search though the commits and code
- [x] Specs and Spinach (fix)
- [ ] Specs and Spinach (new ones)
- [x] Take care of omnibus support for elasticsearch 
- [x] Documentation
- [ ] Add elasticsearch check to GitLab check
- [x] Update and create index means

See merge request !109

Hide issues settings when issues are disabled

This closes gitlab-org/gitlab-ee#198

/cc @DouweM @axil @JobV 

See merge request !122

[ci skip]

[ ci skip]