Migrate MR threads toolbar to GitLab UI

What does this MR do?

For the threads toolbar at the top of merge requests:

• Migrate Bootstrap buttons to GitLab UI
• Remove unused styles and classes
• Improve toggle threads tooltip text to switch between states

Screenshots

	Before	After
LG
SM
XS
All threads resolved
Resolve all threads tooltip
Jump to thread tooltip
Toggle threads tooltip

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Code review guidelines
• Style guides

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

Related to #229325 (closed)