WIP: Integrate YouTrack service

What does this MR do?

Added new issue tracker for youtrack.com to support youtrack-specific issue referencing: {URL}/{PROJECT-KEY}-{ISSUE_ID}

Markdown regex was changed for youtrack-specific project service to catch both project key and issue id to the issue_id field. Note, I just took readmine service as an example and added the same for YouTrack with changed regex. Also, tests and documentation was added in the same way as for readmine service.

What are the relevant issue numbers?

gitlab-org/gitlab-ce#42595

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer