Display tooltip for mentioned users and groups

## What does this MR do?

Display tooltip for mentioned users and groups in Markdown.

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

I don't know.

## Why was this MR needed?

No tooltips were displayed.

## What are the relevant issue numbers?

fixes #17060 

## Screenshots

![user-tooltip](/uploads/501b7d1b2a5e6adcadefbc56b08422ec/user-tooltip.png)

![group-tooltip](/uploads/0152a452f9a1ed9fc86101b242f7e4c2/group-tooltip.png)


See merge request !5261