Add CSV export for First Class Vulnerabilities

What does this MR do?

Related to #197494 (closed)

In this MR we add the ability in API to generate CSV Export for Vulnerabilities as a background job. All exports with files will be removed after 1 hour after they were available for download.

New endpoints:

• POST /api/v4/projects/:project_id/vulnerability_exports - initiate export

{
    "id": 7,
    "created_at": "2020-03-11T21:56:28.835Z",
    "project_id": 19,
    "format": "csv",
    "status": "created",
    "started_at": null,
    "finished_at": null,
    "_links": {
        "self": "http://gitlab.alan:3000/api/v4/projects/19/vulnerability_exports/7",
        "download": "http://gitlab.alan:3000/api/v4/projects/19/vulnerability_exports/7/download"
    }
}

• GET /api/v4/projects/:project_id/vulnerability_exports/:id - get export status

{
    "id": 7,
    "created_at": "2020-03-11T21:56:28.835Z",
    "project_id": 19,
    "format": "csv",
    "status": "finished",
    "started_at": "2020-03-11T21:57:28.835Z",
    "finished_at": "2020-03-11T21:59:28.835Z",
    "_links": {
        "self": "http://gitlab.alan:3000/api/v4/projects/19/vulnerability_exports/7",
        "download": "http://gitlab.alan:3000/api/v4/projects/19/vulnerability_exports/7/download"
    }
}

• GET /api/v4/projects/:project_id/vulnerability_exports/:id/download - download export file

root-security-reports_vulnerability_findings_2020-03-10.csv

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team