Deprecate .toolbar-button-icon in favor of .gl-spinner

What does this MR do?

Replaces the one instance of .toolbar-button-icon with the .gl-spinner component. This should create no visual regression.

CleanShot_2020-06-30_at_22.35.57

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

N/A

Security

N/A

Closes #223733 (closed)