Api doc fix

See merge request !109

Disable retry for old build

https://gitlab.com/gitlab-org/gitlab-ci/issues/87

See merge request !108

Fix OAuth redirect URLs with GitLab using relative_url

### What does this MR do?

This MR fixes URLs used for oauth authentication - current implementation will only with with GitLab instance if it does not use relative_url.

### Are there points in the code the reviewer needs to double check?

No

### Why was this MR needed?

In case of relative_url GitLab installation, OAuth links were removing the relative path part. It was caused by incorrect configuration of OAuth2::Client class.

### What are the relevant issue numbers / Feature requests?

I couldn't find any related issue

See merge request !26

Fixed broken can_test for Slack notifications

Small bug I noticed during e-mail notifications rework

See merge request !29

Receive hostname

Coordinator consumes Runner's hostname and uses it's as a description during registration.

This MR is required to make it work:
https://gitlab.com/gitlab-org/gitlab-ci-runner/merge_requests/10

See merge request !28

Added OAuth2 section to update doc

See merge request !107

Slack specs typo

Fixes failed test introduced by https://gitlab.com/gitlab-org/gitlab-ci/merge_requests/14#note_766148

See merge request !25

Schema update

See merge request !106

Settings enhancements with services transplantation

As discussed here: https://gitlab.com/gitlab-org/gitlab-ci/issues/68 and here https://gitlab.com/gitlab-org/gitlab-ci/issues/63 this is implementation of Services backported from GitLab. As proof of concept Slack Service is implemented. What is missing here is the specs for services and some design choices described in first issue.

See merge request !14

New service sample image

## before
![service_sample](https://gitlab.com/uploads/axil/gitlab-ci/74597df3b0/service_sample.png)

## after

![service_sample](https://gitlab.com/uploads/axil/gitlab-ci/aa0deb5b16/service_sample.png)

See merge request !23