Refactor event item component to use callback functions instead of $emit

What does this MR do?

This MR does the following to the event item component:

1. Use a callback function instead of emitting an event for buttons added to the right-content slot (see comments for more detail).
2. Rename the prop iconStyle to iconClass to better reflect what it's for.
3. Add js-user-link to the user avatar link so that the hover popover works.

NOTE: This MR requires that !28651 (merged) be merged first. Though this MR doesn't depend on any code from that MR, other MRs I currently have open depend on both, so in order to get the diffs to look right, I based this MR off the other one.

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