Jira refactoring to jira gem

part of gitlab-org/gitlab-ee#1008

Refactor the JIRA integration to simplify configuration and use the jira gem instead of doing our own calls.

To Do

• Working implementation
• Finalize error handling
• Write/Fix tests
• Documentation
• Migrations

Screenshots

Old configuration

New configuration

Original MR: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/2572/commits