Add once method to event hubs

What does this MR do?

We recently started to migrate away from Vue event hubs in favor of a mitt-based approach. However, mitt doesn't expose a once method, which can be a problem as we rely on the soon to be deprecated $once method in some places.

This MR adds a .once() method to our mitt event hubs and binds it to $once as well.

An alternative approach would be to use dush instead of mitt as it supports once out-of-the-box.

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

• 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