Add tooltip to user's calendar activities

What does this MR do?

Adds tooltip on hover for user's calendar activitities

Are there points in the code the reviewer needs to double check?

Duplicated code. One way is to add helper method for events, but I'd add unnecessary complexity.

Why was this MR needed?

"Most recent activity" feed's events titles do have tooltips on hover. It'd be consistent if calendar's activites did have tooltip too.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #28925 (closed)