Track usage of merge request file header buttons

What does this MR do?

For issue #197412 (closed)

Following the tracking guide, this merge adds tracking to the following 6 buttons inside a diff_header:

1. Copy file path button
2. Toggle comments for this file button
3. Edit file button
4. Show full file button
5. View file @ SHA button
6. Review app link button

Screenshots

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

As outlined in the frontend tracking guide, I have added a karma test for the copy to clip board button to ensure events are tracked when the button is clicked.

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