Replace `XMLHttpRequest` with `axios` to fetch Balsamiq mockups

What does this MR do?

Replaces use of XMLHttpRequest in app/assets/javascripts/blob/balsamiq/balsamiq_viewer.js with axios.get.

Screenshots

Following is preview of .bmpr file which was loaded via axios.

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.
• Tested in all supported browsers