Implement build trigger API

This commit implements Build Triggers.

There are changes to API request:
- Due to security advised method to pass trigger token is to use form data
- Advised method to pass variables is to use form data

TODO:
- [x] Implement API
- [x] Implement UI
- [x] Dimitriy and Valery review
- [x] Write specs
- [x] Write documentation
- [x] Job documentation review

See merge request !229

Fix navigation icons

See merge request !239

Fix variable saving error

This is regression not allowing to save the variables if the variable name exists in other projects. This also fixes error rendering issue for variables, which previously was rendered on project's settings page.

See merge request !237

Add example for testing clojure application

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

See merge request !238

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

API for trigger when requested returns trigger request with commit information, but without the created builds

Require the variable key to be set and unique

See merge request !234

Add missing stage when doing retry on a build

See merge request !236

Make the YAML validation more stricter

- deny all globals that are not hashes and doesn't contain script, the
  script is required now
- verify that script is either string or array of an strings

See merge request !235

- deny all globals that are not hases and doesn't contain script, the script is required now
- verify that script is either string or array of an strings

Fix typo in modal title

See merge request !233

changes docker image name from python to ruby

See merge request !232

Fix broken yaml errors

It fixes two errors:
1. invalid `.gitlab-ci.yml` did not trigger yaml_errors, thus making the commit success
2. skipped status were used for invalid `.gitlab-ci.yml`

Link to issue: https://gitlab.com/gitlab-org/gitlab-ci/issues/261

/cc @vsizov 

See merge request !228

Support yaml variables

This MR introduces ability to define variables from YAML.

```yaml
variables:
  DB_NAME: postgres

test:
  script: echo $DB_NAME
```

The variables are passed using the same API as Secure Variables. The API introduces additional parameter: public. All variables defined in YAML are marked as public. The GitLab Runner when detects public variables will pass them to the services. This makes it easy to fine tune linked services to for example define database name.

```yaml
services:
- postgres

variables:
  POSTGRES_DB: gitlab
```

The above example will run [postgres](https://registry.hub.docker.com/u/library/postgres/) and pass POSTGRES_DB to postgres container making it to create `gitlab` database instead of default `postges`.

**Note:** All variables will passed to all service containers. It's not designed to distinguish which variable should go where.

/cc @sytses @vsizov @dzaporozhets 

See merge request !227