Update PublishToolbar with GlButton loading flag

What does this MR do?

This MR updates the Static Site Editor's PublishToolbar component to leverage the recently added loading prop of the GlButton in favor of a custom use of the GlLoadingIcon.

This is both a technical cleanup win in addition to a UX win. The latter is the case as the update aligns with "1st generation labeling". This eases a user's understanding of a UI as the visual indication of a change is on/at (1st gen) the point of interaction as opposed to being far away (2nd or 3rd gen) as was the case with the previous loading indicator.

Screenshots

Before	After

---

The "After" render above leads to the current WIP status as there's a small CSS technicality to resolve. As can be seen in the screenshots below, the gitlab/ui class gl-mr-2 isn't loading as expected. - This has since been resolved

Current	Expected

• WIP render issue resolved

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