[Part 5] Terminal sync service status label in IDE status bar

What does this MR do?

Don't merge until => https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14034
CE Port => https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/29371

This MR is part of the larger https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/12239.

This MR refactors the status bar of the IDE so that EE can inject a status for the terminal_sync service.

Screenshots

Connecting	Connected (idle)	Connected + Uploading	Error

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.

Links

• Part of #5276 (closed)