Add Web IDE as dropdown item to diff file edit

What does this MR do?

This MR transforms the edit file button in the MR diffs view to a dropdown that contains

• Edit file
• Edit in Web IDE

Screenshots

	Screenshot
Edit icon is now a dropdown! 🎉
Preserves showing "Fork" callout when user can't edit on source branch.

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

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

References

• Related to #21654 (closed)